LIBRARY 

OF    THE 

UNIVERSITY  OF  CALIFORNIA. 
Cla&s 


QUANTITATIVE 
CHEMICAL   ANALYSIS 

BT 

ELECTROLYSIS 


BY 

PKOF.  ALEXANDER  CLASSEN,  PH.D. 

PRIVY  COUNCILLOR 

Director  of  the  Laboratory  of  Electrochemistry  and  Inorganic  Chemistry  in  the 
Royal  Institute  of  Technology  at  Aachen 


AUTHORIZED   TRANSLATION 

FOURTH  ENGLISH  FROM  THE  FOURTH  GERMAN  EDITION 

REVISED  AND  ENLARGED 

BY 

BERTRAM  B.  BOLTWOOD,  PH.D. 

Formerly  Instructor  in  Physical  and  Analytical  Chemistry  in  the 
Sheffield  Scientific  School  of  Yale  University 


FIRST   THOUSAND 

IE 

-  ITY 

A\^    ' 


JOHN  WILEY   &   SONS 

LONDON  :  CHAPMAN  &  HALL,  LIMITED 

1903 


CG 


GENERAL 


Copyright,  1903, 

BY 

BERTRAM  B.  BOLTWOOD. 


ROBERT  DRUMMOND,    PRINTER,    NEW  YORK. 


AUTHOR'S  PREFACE  TO  FOURTH  GERMAN 

EDITION. 


THE  present  edition,  revised  with  the  assistance  of  Dr. 
Walter  Lob,  differs  from  the  previous  edition  in  having  the 
Introduction  amplified  by  the  insertion  of  a  section  devoted  to 
theory.  This  was  required  the  more  since  recent  investiga- 
tions in  electrochemical  analysis  have  been  directed  to  explain- 
ing the  reactions  taking  place  in  solutions  and  to  determining 
the  magnitudes  of  the  electrical  factors. 

The  importance  of  specific  directions  respecting  the  elec- 
trode potential,  current-strength,  and  decomposition  potential 
has  been  demonstrated.  The  author,  with  the  help  of  his 
assistants,  has  experimentally  determined  these  electrical 
factors,  for  not  only  his  own  methods,  but  also  for  a  consid- 
erable number  of  other  methods,  and  has  incorporated  them 
in  the  text.  Additional  methods  by  other  authors,  in  which 
directions  concerning  these  important  electrical  factors  are 
lacking,  have  been  omitted,  in  view  of  the  fact  that  these 
methods  are  either  uncertain  or  entirely  impractical;  refer- 
ences to  them  will  be  found,  however,  under  the  heading  of 
Literature. 

The  book  has  been  made  the  more  complete  by  the 
descriptions  of  a  variety  of  measuring  instruments,  sources  of 
current  and  other  apparatus,  as  well  as  simple  and  complete 
appliances  for  carrying  out  electrolytic  experiments.  These 
have  been  illustrated  by  a  large  number  of  new  cuts,  in  the 
text  and  in  the  appended  tables. 

A.  CLASSEN. 

AACHEN,  January  18,  1897. 


117280 


TRANSLATOR'S  PREFACE. 


THE  chief  object  in  preparing  the  present  English  edition 
of  this  book  has  been  to  include  a  considerable  number  of  new 
electroanalytical  methods  which  have  been  published  since 
the  appearance  of  the  fourth  German  edition  in  1897. 

This  task  has  been  greatly  simplified  by  the  very  kind 
assistance  of  Professor  Classen,  who  has  generously  placed  his 
new  and  valuable  work  on  "Ausgewahlte  Methoden  der 
Analytischen  Chemie,"  published  in  Berlin  in  1902,  at  the 
disposal  of  the  translator.  This  book  covers  a  wide  field  in 
analytical  chemistry  and  embraces  a  variety  of  special 
subjects.  It  has  been  freely  used  in  preparing  the  present 
English  edition. 

Part  First  of  the  German  original  has  been  divided  into 
two  sections,  and  the  arrangement  of  the  text  has  been 
altered  to  permit  of  a  more  systematic  treatment  of  the 
subject.  Much  new  material  has  been  introduced  into  this 
part,  and  acknowledgment  is  especially  due  to  Professors 
Hastings  and  Beach,  from  whose  Text-Book  of  General  Physics 
many  of  the  descriptions  of  electrical  apparatus  have  been 
taken. 

To  Part  Second  many  new  methods  of  analysis  have  been 
added,  the  source  of  these  being  Professor  Classen's  book 
mentioned  above  and  the  original  papers  in  the  chemical 
journals. 


vi  TRANSLATOR'S  PREFACE. 

The  translator  has  attempted  to  retain  all  of  the  valuable 
material  contained  in  the  fourth  German  edition  and  is  solely 
responsible  for  any  errors  or  mistakes  in  the  new  material 
which  has  been  inserted.  He  desires  to  express  here  his 
thanks  to  Professor  H.  A.  Bumstead  of  the  Sheffield  Scientific 
School  for  his  valuable  advice  and  criticism. 

B.  B.  BOLTWOOD. 

NEW  HAVEN,  Conn.,  April,  1903. 


CONTENTS. 


PART   FIRST. 
SECTION  I.— INTRODUCTORY. 

CHAPTER  PAGE 

I.     HISTORICAL ' 1 

II.     THEORY  OF  SOLUTION 6 

III.  ELECTROLYTES 10 

IV.  CURRENT-STRENGTH,  POTENTIAL 16 

V.     FARADAY'S  LAW 19 

VI.     OHM'S  LAW  .  ., 22 

VII.     MIGRATION  OF  IONS 26 

VIII.     CONDUCTIVITY  OF  SOLUTIONS 30 

IX.     ELECTROLYSIS 34 

X.     ELECTROMOTIVE  FORCE 42 

XI.     POLARISATION  . 47 

SECTION  II.— DESCRIPTIVE. 

XII.     ELECTROCHEMICAL  ANALYSIS 49 

XIII.  DETERMINATION  OF  ELECTRICAL  MAGNITUDES -55 

XIV.  SOURCE  OF  CURRENT 73 

XV.      REGULATING  CURRENT-STRENGTH  AND  POTENTIAL 95 

XVI.      ACCESSORY  APPARATUS 110 

XVII.      THE  ANALYTICAL  PROCESS 128 

XVIII.      ARRANGEMENTS  FOR  ANALYSIS 312 

PART   SECOND. 
SPECIAL. 

SECTION 

I.     QUANTITATIVE  DETERMINATION  OF  METALS. 153 

II.     DETERMINATION  OF  NITRIC  ACID  IN  NITRATES 221 

III.  DETERMINATION  OF  THE  HALOGENS 223 

IV.  SEPARATION  OF  METALS 225 

V.     SEPARATION  OF  THE  HALOGENS 278 

PART    THIRD. 

I.     APPLIED  EXAMPLES  OF  ELECTROCHEMICAL  ANALYSIS  ....  281 

II.     REAGENTS 293 

INDEX '. 297 

vii 


QUANTITATIVE  ANALYSIS  BY  ELECTKOLYSIS, 


PART  FIRST. 
SECTION  I.— INTRODUCTORY. 


CHAPTER   I. 
HISTORICAL. 

THE  development  of  electrochemical  analysis  has  been 
almost  wholly  empirical.  The  most  suitable  conditions  for 
the  quantitative  separation  of  metals  by  electricity  have 
been  determined  from  a  great  number  of  experiments,  con- 
ducted with  diligence  and  perseverance,  while  the  nature  of 
the  reactions  involved  has  not  always  at  the  time  been  clearly 
understood.  The  relatively  recent  development  of  electro- 
chemistry has  served  to  throw  much  light  on  the  theory  of 
quantitative  electrolysis,  and  the  importance  and  significance 
of  the  electrical  factors  and  other  conditions  are  now  much 
more  clearly  understood. 

The  first  attempts  at  the  electrolytic  determination  of 
the  metals  were  entirely  qualitative  in  character.  Shortly 
after  the  discovery,  by  Nicholson  and  Carlisle  (1800),  of  the 
decomposition  of  water  by  the  electric  current,  Cruikshank 


2  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

(1801),  having  observed  the  separation  of  metallic  copper, 
suggested  that  the  galvanic  current  might  be  used  for  the 
qualitative  determination  of  other  metals.  This  suggestion 
awakened  but  little  interest.  In  1812  Fischer  employed 
an  electrolytic  method  for  identifying  arsenic  in  animal  fluids, 
and  later,  in  1840,  Cozzi  used  a  similar  method  for  the  de- 
tection of  metals  in  general  in  such  solutions. 

The  discovery  of  galvanoplasty,  a  most  important  techni- 
cal process  closely  allied  to  electrochemical  analysis,  dates 
from  1839  and  was  made  by  Jacobi. 

Gaultier  de  Claubry,  in  1850,  recommended  the  use  of  the 
electric  current  for  detecting  poisonous  metals  in  mixtures 
containing  organic  substances,  and  in  1860  Bloxam  continued 
this  work  and  devised  numerous  methods  by  which  he  at- 
tempted to  make  the  identification  of  arsenic  and  antimony 
possible  in  the  presence  of  other  metals.  In  this  work  he 
was  assisted  somewhat  by  the  directions  for  the  separation 
of  metals  from  mixtures  published  by  Morton  in  1851. 

Becquerel  observed,  as  early  as  1830,  that  lead  and 
manganese  often  separated,  not  as  metals  at  the  negative 
pole,  but  in  the  form  of ,  oxides  on  the  positive  pole,  a  property 
which  permitted  these  metals  to  be  readily  separated  from 
others.  Investigations  on  the  qualitative  decomposition 
of  inorganic  salts  of  the  metals  were  also  carried  out  by 
Despretz  (1857),  Nickles  (1862),  and  Wohler  (1868).  The 
work  of  A.  C.  and  E.  Becquerel  (1862)  on  the  electrolytic 
reduction  of  the  metals  was  likewise  of  an  entirely  qualitative 
character. 

It  can  be  readily  understood  that  with  such  abundant 
data  at  hand  the  development  of  quantitative  electrolysis 
was  comparatively  rapid. 

The  field  of  quantitative  investigation  was  first  opened 
by  W.  Gibbs  (1864),  who  carried  out  an  investigation  on  the 


HISTORICAL,  3 

electrolytic  determination  of  copper  and  nickel,  which  in- 
cluded a  description  of  the  methods  for  the  determination  of 
silver  and  bismuth  in  the  form  of  metals,  as  well  as  of  lead 
and  manganese  in  the  form  of  peroxides.  He  also  published 
studies  on  the  separation  of  zinc,  nickel,  and  cobalt.  The 
possibility  of  the  quantitative  determination  of  copper  was 
confirmed  by  Luckow  (1865),  who  had  worked  at  it  for  a 
number  of  years.  The  quantitative  electrolytic  determina- 
tion of  metals  was  entitled  by  him  ' i  electro-metal-analysis. ' ' 
This  author  published  at  the  same  time  a  series  of  directions 
for  the  method  of  using  the  current  for  analytical  work,  and 
by  these  precise  instructions  laid  the  foundation  for  many 
later  researches. 

The  attention  of  investigators  was  then  directed  principally 
to  the  chemical  reactions  which  took  place  when  different 
sources  of  current  were  used  and  when  the  other  physical 
conditions  were  varied.  The  salts  of  the  metals  and  the 
solvents  suitable  for  use  and  the  proper  substances  to  be 
added  to  the  solutions  were  investigated  and  determined. 
Wrightson  (1876)  called  attention  to  the  fact  that  the  accu- 
racy of  copper  determinations  was  influenced  by  the  presence 
of  other  metals  and  ascertained  the  limits  under  which  copper 
could  be  accurately  determined  in  the  presence  of  antimony. 

Simultaneous  with  the  announcement  of  the  electrolytic 
determination  of  gallium  in  alkaline  solutions  by  Lecoq  de 
Boisbaudran  (1877)  came  the  announcement  by  Parodi  and 
Mascazzini  that  zinc  could  be  determined  in  a  solution  of  its 
sulphate  to  which  an  excess  of  ammonium  acetate  had  been 
added,  and  that  metallic  lead  could  be  quantitatively  pre- 
cipitated from  an  alkaline  tartaric  acid  solution  containing 
an  alkali  acetate. 

We  are  indebted  to  Bichert  (1878)  for  the  first  accurate 
directions  for  the  determination  of  manganese.  He  ob- 


4  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

served  that  this  element  may  be  completely  separated  at  the 
positive  pole  in  the  form  of  an  oxide  from  solutions  of  the 
nitrate.  This  property  permits  of  the  electrolytic  separation 
of  manganese  from  other  metals,  e.g.,  copper,  cobalt,  nickel, 
sine,  etc. 

Other  papers  which  were  published  at  that  time  by 
Luckow,  F.  W.  Clarke,  and  J.  B.  Haunay  described  the 
electrolytic  determination  of  mercury,  which  was  found  to 
separate  readily  from  solutions  of  the  chloride  and  sulphate. 

A  method  for  the  electrolytic  determination  of  cadmium 
was  found  by  F.  W.  Clarke  (1878),  who  succeeded  in  precipitat- 
ing this  metal  from  solutions  of  its  acetate,  and  Yver  (1880) 
employed  a  similar  solution  for  separating  cadmium  from  zinc. 

The  determination  of  zinc  from  solutions  of  the  double 
cyanides  was  carried  out  by  Beilstein  and  Jawein  (1879),  and 
Fresenius  and  Bergmann  (1880)  successfully  precipitated 
metallic  nickel  and  cobalt  from  solutions  containing  an  excess 
of  free  ammonia  and  ammonium  sulphate. 

Edgar  F.  Smith  showed  (1880)  that  if  uranium  acetate 
solutions  were  electrolysed  the  uranium  was  completely 
precipitated  as  a  hydrated  protosesquioxide ;  and,  further, 
that  molybdenum  could  be  deposited  as  hydrated  sesqui- 
oxide  from  warm  solutions  of  ammonium  molybdate  in  the 
presence  of  free  ammonia.  We  are  indebted  to  the  same 
author  and  his  students  for  a  large  number  of  valuable  con- 
tributions to  the  literature  of  electrochemical  analysis. 

Luckow  (1880)  rendered  a  special  service  in  the  publica- 
tion of  his  observations  on  the  reactions  which  take  place 
during  electrolysis.  He  pointed  out  the  reduction  from 
higher  to  lower  states  of  oxidation  in  the  case  of  chromic 
acid,  iron,  and  uranium  salts,  and  demonstrated,  on  the  other 
hand,  that  sulphites  and  thiosulphates  are  oxidised  to  sul- 
phates. He  summed  up  the  results  of  his  observations  in  a 


HISTORICAL.  5 

law,  that  in  general  the  electric  current  exerts  a  reducing 
action  on  acid,  and  an  oxidising  action  on  alkaline,  solutions. 
Recent  investigations  have  shown,  however,  that  other  factors 
are  of  importance  in  these  reactions. 

In  the  year  1881  Alexander  Classen  and  his  students 
began  a  series  of  investigations  on  quantitative  analysis  by 
electrolysis  which  ultimately  included  nearly  all  of  the  metals. 
It  was  he  who  first  pointed  out  the  value  of  oxalic  acid  and 
the  double  oxalates.  A  large  number  of  electrolytic  methods 
originated  by  him  will  be  described  in  this  book. 

At  about  the  same  time  as  and  quite  independently  of 
Classen,  Reinhardt  and  Ihle  proposed  the  double  oxalates 
for  the  electrolytic  determination  of  zinc. 

An  attempt  was  made  (1880)  by  Gibbs,  who  used  a  mer- 
cury cathode,  to  determine  metals  by  observing  the  increase 
in  weight  of  the  mercury  due  to  the  formation  of  an  amalgam, 
and  a  similar  method  was  employed  by  Luckow  (1886)  for 
the  determination  of  zinc. 

Since  the  year  1886  a  great  number  of  publications  on 
electrochemical  analysis  have  appeared,  and  the  most  im- 
portant of  these  will  be  mentioned  later. 

Especially  worthy  of  mention  at  this  point,  however,  are 
the  experiments  conducted  by  Vortmann  (1894)  on  the  elec- 
trolytic determination  of  the  halogens. 

The  investigations  of  Kiliani  (1883),  on  the  significance  of 
the  potential-difference  in  electrolytic  determinations,  served 
to  draw  attention  to  this  important  factor,  and  the  later  work 
of  Le  Blanc  (1889)  on  the  potential-differences  necessary 
for  the  decomposition  of  solutions  of  the  salts  of  various 
metals  added  greatly  to  the  available  theoretical  data.  In 
1891  Freudenberg  successfully  separated  a  number  of  metals 
from  solutions  containing  several  by  carefully  regulating  the 
potential-difference  of  the  current  which  he  employed. 


CHAPTER  II. 
THEORY  OF  SOLUTION. 

THE  modern  theory  of  solution  is  the  foundation  of  the 
science  of  electrochemistry.  It  is  therefore  most  essential 
that  this  theory  should  be  clearly  understood  by  all  workers 
in  this  branch  of  chemical  science. 

Until  recent  years  solutions  were  considered  to  be  mere 
mechanical  mixtures  of  solvent  and  solute  and  no  general 
laws  governing  such  mixtures  had  been  discovered.  A 
theory  assuming  chemical  interaction  between  solvent  and 
solute,  the  so-called  hydrate  theory,  involving  the  chemical 
combination  of  the  molecules  of  the  solute  with  the  mole- 
cules of  water,  was  proposed,  but  since  this  theory  did  not 
prove  to  be  a  satisfactory  working  hypothesis  it  was  grad- 
ually abandoned. 

The  phenomenon  of  diffusion  was  well  known.  This  is 
exhibited  when  solutions  of  dissolved  substances  are  placed 
in  contact  with  the  pure  solvent.  In  such  cases  the  dissolved 
substance  gradually  works  its  way  from  the  stronger  solution 
through  the  entire  solvent  until  finally  after  sufficient  time 
has  elapsed  the  mixture  of  solvent  and  solute  is  found  to  be 
absolutely  uniform  and  all  portions  of  the  solution  are  of 
uniform  concentration.  This  behavior  of  dissolved  sub- 
stances suggests  the  existence  of  a  force  tending  to  drive  the 
particles  out  into  the  adjoining  solvent,  and  in  1877  Pfeffer* 

*  Osmotische  Untersuchungen.     Leipzig  1877. 


THEORY    OF    SOLUTION.  7 

showed  by  a  series  of  experiments  that  when  the  dissolved 
substance  was  prevented  from  diffusing  into  the  solvent  a 
pressure  of  considerable  magnitude  was  exerted  upon  the 
retarding  membranes  which  he  employed.  One  of  the  mem- 
branes which  he  used  was  copper  ferrocyanide  precipitated 
in  the  walls  of  a  porous  earthenware  cylindrical  jar.  This 
membrane  allows  the  water  used  as  a  solvent,  but  not  the 
substance  contained  in  the  solution,  to  pass  through  it  (semi- 
permeable  membrane),  and  by  placing  a  solution  in  the  jar 
which  was  surrounded  by  pure  water,  he  was  able  to  measure 
approximately  the  pressure  which  was  exerted. 

This  pressure  is  known  as  the  osmotic  pressure  of  the 
substance  in  solution,  and  as  a  result  of  his  experiments 
Pfeffer  reached  the  following  conclusions: 

1.  That  the  pressure  is  dependent  on  the  nature  of  the 
dissolved  substance. 

2.  That  for  any  given  substance  the  pressure  depends 
on  the  concentration  of  the  solution  and  is  in  direct  propor- 
tion to  this. 

3.  That  the  pressure  at  a  given  concentration  is  depend- 
ent on  the  temperature,  and  shows  a  regular  increase  with 
rising  temperature. 

Pfeffer  also  concluded  that  the  magnitude  of  the  pressure 
was  influenced  by  the  nature  of  the  membrane,  but  this 
assumption  was  later  shown  to  be  erroneous. 

Pfeffer's  investigations  attracted  but  little  attention  at 
the  time  they  were  published.  It  was  not  until  the  year 
1885  that  their  important  bearing  on  the  theory  of  solution 
was  appreciated. 

In  1885  Van't  Hoff  called  attention  *  to  the  fact  that  there 


*  Lois  de  1'Equilibre  Chimique.      Memoire  presente  .a  1'  Academic 
des  Sciences  de  Suede  le  14  Octobre  1885. 


8  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

seemed  to  be  a  very  close  and  .striking  relation  between  the 
laws  of  gas  pressure  and  the  laws  of  osmotic  pressure,  and 
showed  that  the  osmotic  pressures  of  the  dilute  solutions 
measured  by  Pfeffer  could  be  calculated  from  the  gas  laws 
alone,  the  values  thus  obtained  corresponding  within  the 
limits  of  experimental  error  with  the  values  measured  by 
Pfeffer.  He  thus  demonstrated  that  substances  in  dilute 
solutions  have  an  osmotic  pressure  which  is  equal  to  the 
pressure  which  they  would  exert  if  they  were  in  a  gaseous 
form  at  the  same  temperature  and  occupied  under  these 
conditions  a  volume  equal  to  the  volume  of  the  solution. 
Not  only  does  the  osmotic  pressure  vary  inversely  as  the 
volume  (Boyle's  law),  but  the  osmotic  pressure  is  also  directly 
proportional  to  the  absolute  temperature  (Gay-Lussac's  law). 

If  it  be  assumed,  therefore,  that  the  laws  of  gases  apply 
generally  to  substances  in  solution,  Avogadro's  hypothesis 
may  be  applied  in  the  following  form : 

Equal  volumes  of  solutions  of  different  substances  at  the 
same  temperature  and  having  the  same  osmotic  pressure 
contain  an  equal  number  of  molecules.  This  is  known  as 
Van't  Hoff's  law  for  solutions. 

This  law  furnishes  a  valuable  means  for  determining 
the  molecular  weight  of  chemical  compounds.  It  is  only 
necessary  to  determine  the  osmotic  pressure  and  temperature 
of  a  solution  containing  a  known  weight  of  the  compound 
in  a  given  volume  of  solution.  From  the  data  thus  obtained 
the  molecular  weight  of  the  substance  in  solution  can  be 
readily  calculated. 

Since  the  direct  measurement  of  the  osmotic  pressure 
is,  for  various  reasons,  a  very  difficult  operation  it  is  seldom 
resorted  to  in  practice.  Indirect  methods  which  are  more 
convenient  are  employed  instead.  These  methods  are  based 
on  the  determination  of  other  properties  of  solutions  which 


THEORY    OF    SOLUTION.  9 

show  a  direct  variation  with  changes  in  the  osmotic  pres- 
sure. Chief  among  these  indirect  methods  are  those  which 
depend  on  the  measurement  of  the  depression  of  the  freezing 
point,  the  elevation  of  the  boiling  point,  and  the  lowering  of 
the  vapor  pressure  of  any  pure  solvent  caused  by  the  intro- 
duction of  a  known  weight  of  any  soluble  substance.  These 
are  all  directly  proportional  to  the  osmotic  pressure  of  the 
substance  in  solution. 


CHAPTER  III. 

ELECTROLYTES. 

THE  development  of  the  theory  of  osmotic  pressure 
brought  to  light  the  fact  that  a  great  number  of  chemical 
compounds  when  dissolved  in  water  exerted  osmotic  pres- 
sures which  did  not  agree  with  those  which  would  be  expected 
from  Van't  HofPs  law  alone.  These  compounds,  among  which 
were  included  most  of  the  substances  used  as  reagents  in 
analytical  chemistry,  could  be  divided  into  three  general 
classes,  i.e.,  acids,  bases,  and  salts. 

These  apparent  exceptions  to  the  law  were  raised  as 
objections  to  its  adoption,  just  as  the  abnormal  gas  density 
of  ammonium  chloride,  before  this  was  fully  understood, 
was  considered  a  proof  of  the  fallacy  of  Avogadro's  hy- 
pothesis. 

Arrhenius  *  was  the  first  to  offer  a  satisfactory  explanation 
of  the  cause  of  these  abnormal  osmotic  pressures. 

The  theory  proposed  by  him  in  1887  may  be  stated  as 
follows: 

When  a  solid  compound  soluble  in  water  is  introduced 
into  this  liquid  it  passes  into  solution  in  the  form  of  mole- 
cules. If  the  behavior  of  the  compound  is  perfectly  normal, 
e.g.  if  it  gives  an  osmotic  pressure  which  agrees  with  Van't 
Hoff's  law,  the  molecules  undergo  no  further  alteration,  but 
exist  as  such  in  the  solution.  If,  however,  the  substance 

*Zeit.  f.  phys.  Chem.,  1,  631  (1887). 


ELECTROLYTES.  11 

belongs  to  that  class  of  bodies  which  give  abnormal  osmotic 
pressures,  then,  immediately  on  passing  into  solution,  some 
of  the  molecules  dissociate  into  other  particles  which  .are 
called  ions.  These  ions,  which  may  be  either  single  atoms 
or  groups  of  atoms,  have  the  same  effect  on  the  osmotic 
pressure  as  undissociated  molecules.  As  a  result  of  this 
increase  in  the  number  of  particles  in  the  solution  the  osmotic 
pressure  is  greater  than  if  no  dissociation  had  taken  place. 

The  ratio  of  the  number  of  dissociated  molecules  to  the 
total  number  of  molecules  introduced  into  the  solution  is 
called  the  degree  of  dissociation. 

If  one  gram  molecule  of  a  substance  the  composition  of 
which  is  represented  by  AB  is  dissolved  in  a  definite  volume 
of  solvent,  and  if  this  substance  dissociates  into  two  ions,  A 
and  B,  the  degree  of  dissociation  being  equal  to  x,  the  state 
of  the  substance  in  solution  will  be  represented  by  the  fol- 
lowing expression: 


where  1  represents  the  gram  molecule  taken. 

For  a  given  solution  having  a  known  osmotic  pressure 
the  degree  of  dissociation  can  be  calculated  from  the  equation 


?(*-!)' 

in  which  P  stands  for  the  osmotic  pressure  measured,  p  the 
theoretical  osmotic  pressure  calculated  from  the  gas  laws,  and 
k  the  number  of  ions  into  which  each  molecule  dissociates. 

The  maximum  value  which  x  can  attain  is  unity.  This 
is  its  value  when  all  of  the  substance  contained  in  the  solution 
is  in  the  form  of  ions. 

Several  very  important  points  with  respect  to  the  values 
of  x  have  been  brought  out  by  experiment. 


12  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

1.  The  degree  of  dissociation  for  a  given  substance  dis- 
solved in  water  is  the  same  for  all  solutions  of  the  same  sub- 
stance at  the  same  concentration  and  temperature,  i.e.,  the 
degree  of  dissociation  for  a  given  solution  at  constant  tem- 
perature is  a  constant. 

2.  On  diluting  a  solution  the  degree  of  dissociation  in- 
creases until  the  maximum  value  is  attained.      Beyond  this 
point  further  dilution  produces  no  change  in  the  state  of  the 
dissolved  substance. 

3.  Strong   acids,   strong  bases   and  their   salts   even  in 
fairly   concentrated  solutions  are  almost  completely  disso- 
ciated into  their  ions. 

4.  The   degree   of  dissociation   determined  by  measure- 
ments of  the  osmotic  pressure  or  by  any  of  the  indirect 
methods  already  mentioned  is  found  to  agree  exactly  with 
the  degree  of  dissociation  as  determined  by  an  entirely  sepa- 
rate and  independent  method  depending  upon  the  electrical 
conductivity  of  the  solution  (see  p.  32). 

A  chemical  compound  which  in  a  dissolved  or  melted 
condition  conducts  the  electric  current  is  called  an  electrolyte. 
If  an  electric  current  is  passed  through  the  aqueous  solution 
of  an  electrolyte,  certain  chemical  changes  are  produced. 
The  process  is  called  electrolysis.  The  points  at  which  the 
current  enters  and  leaves  the  solution  are  called  the 
electrodes. 

Arrhenius  called  attention  to  the  fact  that  all  solutions 
which  contain  dissociated  substances  have  the  property  of 
conducting  the  electric  current,  indeed  the  greater  the  degree 
of  dissociation  the  better  the  conductivity  of  the  solution, 
while  this  property  is  not  possessed  to  an  appreciable  extent 
by  solutions  of  substances  which  correspond  to  Van't  Hoff's 
law. 

He  therefore  assumed  that  the  undissociated  molecules 


ELECTROLYTES.  13 

in  a  solution  take  no  part  in  conducting  the  current  and  that 
the  conductivity  of  the  solution  is  due  to  the  ions  alone. 

This  view  is  borne  out  particularly  by  the  fact  that  the 
conductivity  of  a  solution  per  molecule  of  dissolved  electro- 
lyte increases  with  the  dilution;  namely,  with  increased 
dissociation. 

In  order  to  explain  the  property  of  conductivity,  as 
well  as  other  properties,  the  following  conditions  have  been 
assumed : 

1.  That  the  separate  ions  are  charged  with  electricity. 

2.  That  a  molecule  is  dissociated  into  two  different  kinds 
of  ions,  one  kind  being  charged  positively,  the  other  nega- 
tively. 

3.  That  the  sum  of  the  negative  charges  borne  by  the 
negative  ions  is  exactly  equal  to  the  sum  of  the  positive 
charges  borne  by  the  positive  ions  formed  from  the  same 
molecule. 

4.  That  the  charges  are  inseparable  from  the  ions  as  such 
and  appear  at  the  very  instant  of  dissociation. 

5.  That  the  composition  of  the  ions  is  similar  to  that  of 
the   substances   which   primarily  appear  at  the   electrodes 
when  the  solution  is  submitted  to  electrolysis. 

Since  bodies  charged  with  electricity  of  unlike  sign  are 
known  to  attract  each  other,  while  the  opposite  is  true  for 
those  which  bear  similar  charges,  the  ions  which  appear  at 
the  negative  electrode  (cathode)  are  assumed  to  be  charged 
positively  (cathions)  and  those  which  appear  at  the  positive 
electrode  (anode)  are  assumed  to  be  negatively  charged 
(anions). 

In  the  table  given  on  p.  14  it  is  shown  into  what  ions 
some  of  the  more  common  acids,  bases,  and  salts  dissociate. 
The  nature  of  the  charge  of  each  ion  is  denoted  by  a  sign 
( +  or  - )  placed  above  it. 


14  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

TABLE. 


Acids. 

Ions.                                 Bases. 

Ions. 

HC1 

H,  Cl                  NaOH 

Na,  0~H 

HN03 

H,  N03               KOH 

K,  OH 

HBr 

H,  Br                  Ca(OH)2 

Ca,  OH,  0~H 

CH3COOH 

H,  CH3COO       Ba(OH)3 

Ba,  OH,  OH 

H2S04 

H,  H,  S04* 

H2S03 

H,  H,  SO, 

(COOH)2 

H,  H,  CA 

H3As04 

[Salts. 

H,  H,  H,  As04 

Ions. 

NaCl 

Na,  Cl 

NaN03 

Na,  N03 

KBr 

K,  Br 

CaCl2 

Ca,  Cl,  Cl 

Na,S04 

Na,  Na,  S04 

(COONa)2 

Na,  Na,  C~04 

ZnS04 

Zn,  S04 

K3As04 

K,  K,  K,  A~k)4 

K4Fe(CN)6 

K,  K,  K,  K,  Fe(CN)6 

HNaS04 

H,  Na,  S~04 

All   acids   dissociate  into   hydrogen   cathions   and   acid 
radical  anions,  all  bases  into  metal  cathions  and  hydroxyl 
.  anions,  and  all  neutral  salts  into  metal  cathions  and  acid 

*  In  concentrated  solutions:   H,  HSO4. 


ELECTROLYTES.  15 

radical  anions.  Acid  salts  furnish  both  metal  and  hydrogen 
cathions. 

To  one  who  meets  the  suggestion  for  the  first  time  it 
may  appear  surprising  that  separate  particles  of  sodium 
and  chlorine  can  exist  free  in  an  aqueous  solution  of  sodium 
chloride,  since  metallic  sodium  reacts  so  violently  with 
water  and  with  gaseous  chlorine.  But  these  reactions  are 
between  molecular  sodium  and  water  on  the  one  hand,  and 
between  molecular  sodium  and  molecular  chlorine  on  the 
other. 

When  molecular  quantities  of  metallic  sodium  and  gas- 
eous chlorine  combine  to  form  sodium  chloride  a  considerable 
quantity  of  energy,  which  usually  appears  in  the  form  of 
heat,  is  set  free.  The  reaction  is 

Na2+Cl2  =  2NaCl  (solid)  +   •   •   •  97.6  Cal. 

The  solid  sodium  chloride  possesses  none  of  the  properties 
of  either  metallic  sodium  or  gaseous  chlorine,  but  nevertheless 
it  is  considered  to  contain  the  quantities  of  these  elements 
originally  taken.  When  dissolved  in  water  the  sodium 
chloride  splits  up  into  sodium  ions  and  chlorine  ions,  these 
ions  resembling  sodium  atoms  and  chlorine  atoms  in  no  par- 
ticular except  that  of  composition. 

A  satisfactory  explanation  of  the  cause  for  the  difference 
in  the  properties  of  atomic  sodium  and  chlorine  and  the 
same  elements  in  the  form  of  ions  lies  in  the  fact  that  the 
quantity  of  energy  which  is  associated  with  the  former  is 
greater  than  that  associated  with  equivalent  quantities  of 
the  latter.  It  is  reasonable  to  assume  that  the  energy  which 
is  set  free  when  molecular  sodium  and  chlorine  enter  into 
chemical  combination  results  from  the  loss  of  energy  by 
both  of  these  elements. 


CHAPTER  IV. 
CURRENT-STRENGTH,  POTENTIAL. 

SUPPOSE  that  two  electrodes  composed  of  a  metal  which 
undergoes  no  alteration  during  the  subsequent  action  are 
immersed  in  the  solution  of  an  electrolyte,  and  that  the 
electrodes  are  connected  with  the  positive  and  negative  poles  of 
a  suitable  source  of  current.  Under  these  conditions  the  elec- 
trodes will  be  charged  positively  and  negatively  respectively. 
The  negatively  charged  ions  in  the  solution  will  be  attracted 
to  the  anode  and  the  positively  charged  ions  to  the  cathode, 
and  on  coming  into  contact  with  the  electrodes  the  ions  will 
receive  from  them  equivalent  quantities  of  electricity  of 
a  sign  opposite  to  that  which  they  bear,  that  is,  the  positive 
ions  will  receive  negative  charges  equal  to  the  positive 
charges  which  they  possess,  and  correspondingly  the  nega- 
tive ions  will  receive  positive  charges  from  the  anode  equiv- 
alent to  the  negative  charges  of  which  they  are  the  carriers. 
This  neutralisation  of  the  charges  of  the  ions  having  taken 
place  these  particles  instantly  acquire  the  properties  of  atoms 
or  atomic  radicals. 

As  a  result  of  the  process  described,  electricity  will  dis- 
appear from  both  of  the  electrodes  and  the  effect  on  these 
will  be  comparable  to  that  which  would  take  place  if  they 
were  connected  with  each  other  by  means  of  a  metallic  con- 
ductor. 

If  the  wire  connecting  one  of  the  electrodes  to  the  source 
of  current  be  considered,  then  as  electricity  disappears  from 
the  electrode,  as  a  result  of  the  neutralisation  of  the  charges 

16 


CURRENT-STRENGTH,  POTENTIAL.  17 

of  the  ions,  more  electricity  will  flow  along  the  wire  to  take 
its  place,  and  this  movement  of  electricity  will  continue  so 
long  as  there  are  ions  in  the  solution. 

Such  a  movement  of  electricity  through  a  conductor  is 
called  a  current,  and  since  it  is  evident  that  certain  quantities 
of  electricity  must  pass  along  the  wire  in  a  given  time,  it  is 
possible  to  speak  of  the  strength  of  the  current. 

The  current-strength  in  any  conductor  is  the  quantity 
of  electricity  passing  any  cross-section  in  the  unit  time  (per 
second). 

A  peculiar  significance  is  attached  to  the  term  * '  strength 
of  current"  if  the  flow  of  both  positive  and  negative  elec- 
tricity along  the  conductor  be  considered.  Thus,  for  ex- 
ample, at  the  surface  of  the  positive  electrode,  where  the 
contact  of  the  negative  ions  takes  place,  two  conceptions 
of  the  passage  of  electricity  are  permissible.  It  may  be 
considered  that,  on  the  one  hand,  in  one  second  a  certain 
quantity  of  positive  electricity  n,  exactly  equivalent  to  the 
quantity  of  negative  electricity  nf  borne  by  the  discharging 
ions,  passes  from  the  electrode  to  the  ions,  and  produces 
their  electrical  neutrality;  or,  on  the  other  hand,  the  view 
may  be  taken  that  in  the  same  time  a  quantity  of  positive 

electricity  equal  to  •»  passes  from  the  electrode  to  the  ions 
while  simultaneously  a  quantity  of  negative  electricity  equal 

to  -  -  passes  from  the  ions  to  the  electrode.     In  the  latter 
ft 

case  the  effect  is  the  same  as  in  the  former,  since  a  quantity 
of  positive  electricity  equal  to  n  disappears  from  the  elec- 
trode, and  the  current-strength,  i.e.,  the  total  quantity  of 
electricity  passing  this  cross-section,  is  equal  to  n  in  both 
cases. 

For  the  purpose    of    generality,  therefore,  the   current- 


18  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

strength  through  any  cross-section   of  a  conductor  will  be 
considered  as  equal  to  N  as  defined  by  the  following  equation: 


n  being  the  quantity  of  positive  electricity  passing  in  the 
unit  time,  and  n'  the  quantity  of  negative  electricity  pass- 
ing in  the  same  period  but  in  an  opposite  direction. 

In  order  that  electricity  can  flow  from  one  point  to  an- 
other a  difference  of  electrical  pressure  between  the  two 
points  is  necessary.  This  electrical  pressure  is  called  'poten- 
tial and  the  movement  of  electricity  along  the  conductor  is 
said  to  be  due  to  a  difference  of  potential.  Potential  bears  a 
striking  resemblance  to  pressure  in  fluids  and  to  tempera- 
ture in  heat,  but  the  analogy  is  not  rigid  and  care  must  be 
taken  that  they  are  not  confounded.  Since  two  kinds  of 
electricity  are  recognised  it  is  important  to  remember  that 
while  positive  electricity  moves  from  a  point  of  higher  to  a 
point  of  lower  potential,  negative  electricity  always  moves 
in  the  opposite  direction,  i.e.  from  a  point  of  lower  to  a  point 
of  higher  potential. 


CHAPTER  V. 

FARADAY'S  LAW. 

THIS  law,  which  applies  to  the  passage  of  electricity 
through  the  solution  of  an  electrolyte,  includes  two  propo- 
sitions : 

1.  The  weights  of  the  ions  which  separate  at  an  electrode 
during  equal  intervals  of  time  are  directly  proportional  to 
the  current-strength. 

2.  The  current-strength  remaining  constant,  the  weights 
of  different  ions  which  separate  at  the  electrodes  in  equal 
intervals  of  time  are  in  clirect  proportion  to  the  chemical 
equivalent  weights  of  the  ions. 

The  truth  of  the  first  proposition  can  be  readily  demon- 
strated by  electrolysing  a  copper  sulphate  solution  for  a 
certain  length  of  time  with  a  current  of  given  strength  and 
determining  the  weight  of  the  separated  copper.  If  the 
experiment  is  repeated,  but  with  a  current  of  twice  the  for- 
mer strength,  the  weight  of  the  copper  will  be  twice  that 
precipitated  in  the  first  experiment. 

The  truth  of  the  second  proposition  can  be  similarly 
shown  by  passing  currents  of  equal  strength  through  solu- 
tions of  different  electrolytes  for  equal  periods  of  time.  If 

+     +     + 

a  series  of  solutions  containing  the  ions  H,  Ag,  Cu  (cuprous), 
4-  + 

Cu  (cupric),  Fe  (ferric),  Cl,  and  Br  are  electrolysed  with 

19 


20  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  same  current  for  the  same  length  of  time,  it  will  be  found 
that  the  weights  of  the  elements  set  free  at  the  electrodes 
are  in  the  ratio 

1:107.9:63.6:^^:  ~:  35.5:80, 

Z          o 

respectively. 

These  two  propositions  lead  to  the  following  assumptions: 

1.  That  the  quantities  of  electricity  borne  by  equal  weights 
of  similar  ions  are  equal. 

2.  That  the  quantity  of  electricity  borne  by  any  ion  is 
exactly  equal  to  the  charge  borne  by  every  other  similar  ion. 

3.  That  the  magnitude  of  the  charge  borne  by  any  uni- 
valent  ion  is  the  same  as  that  borne  by  every  other  univalent 
ion. 

4.  That  the  quantity  of  electricity  associated  with  every 
ion  is  directly  proportional   to  the  valence. of  the  ion,  the 
quantity  associated  with  a  bivalent  ion  being  twice  as  great 
as  that  of  a  univalent  ion,  etc. 

The  unit  quantity  of  electricity  has  been  chosen  as  that 
quantity  of  electricity  equivalent  to  the  charge  borne  by 
1.118  milligrams  *  of  silver  ions.  This  quantity  of  elec- 
tricity is  called  a  coulomb. 

A  current  is  of  unit  strength  when  the  quantity  of  elec- 
tricity (p.  18)  which  passes  a  cross-section  of  the  conductor 
in  one  second  is  equal  to  one  coulomb.  The  name  for  the 
unit  of  current-strength  is  the  ampere. 

The  magnitude  of  the  charge  borne  by  a  gram-equivalent 
of  ions  can  be  readily  calculated  from  the  definition  of  the 
coulomb.  It  is  only  necessary  to  divide  the  chemical  equiv- 


*  After  a  careful  study  of  the  results  obtained  by  different  investigators, 
Richards  and  Heimrod  (Zeit.  phys.  Chem.,  41,  302  (1902))  have  decided 
that  the  most  probable  value  is  1.1175  mg. 


FARADAY  S    LAW. 


21 


alent  weight  of  silver,  107.9,  by  0.001118.  The  result  ob- 
tained is  approximately  96,540.*  This  quantity  of  elec- 
tricity is  denoted  by  the  symbol  F  and  may  also  be  defined 
as  that  quantity  of  electricity  required  for  the  discharge,  or, 
better,  for  the  electrical  neutralisation,  of  one  gram-equiva- 
lent of  ions  at  the  electrodes. 

The  weight  (in  milligrams)  of  the  different  ions  set  free 
per  second  at  the  electrodes  by  the  passage  of  a  current  of 
one  ampere  is  known  as  the  electrochemical  equivalent  of 
the  ions.  A  table  of  the  electrochemical  equivalents  of  a 
number  of  ions  is  given  below. 


Element. 

Atomic 
Weight. 

Ion. 

Valence. 

Weight  of  Ions  Discharged  by 
Current  of  One  Ampere 

in  one  sec- 
ond, milli- 
grams. 

in  one  min- 
ute, milli- 
grams. 

in  one 
hour, 
grams. 

Antimony 

120.2 

112.4 
35.45 
63.6 

55.9 

206.9 
200.0 

58.7 
107.93 
119.0 

65.4 

Sb'" 
Sb"" 
Cd" 
Cl' 
Cu' 
Cu" 
Fe" 
Fe'" 
Pb" 
Hg7 
Hg" 
NP' 
Ag' 
Sn" 
Sn"" 
Zn" 

3 
.5 
2 
1 
1 
2 
2 
3 
2 
1 
2 
2 
1 
2 
4 
2 

0.414 
0.249 
0.582 
0.367 
0.659 
0.329 
0.289 
0.193 
1.072 
2.072 
1.036 
0.304 
1.118 
0.616 
0.308 
0.339 

24.86 
14.92 
34.93 
22.03 
39.53 
19.76 
17.37 
11.58 
64.30 
124.32 
62.16 
18.24 
67.08 
36.98 
18.49 
20.32 

1.492 
0.895 
2.096 
1.322 
2.372 
1.186 
1.042 
0.695 
3.858 
7.459 
3.730 
1.094 
4.025 
2.219 
1.109 
1.219 

Cadmium 

Chlorine. 

Cooper 

Iron.  

Lead  

Nickel 

Silver. 

Tin  

Zinc  

96,580  according  to  Richards  and  Heimrod,  loc.  cit 


CHAPTER  VI. 
OHM'S  LAW. 

THE  basis  of  this  law  as  first  discovered  by  Ohm  may  be 
stated  as  follows: 

In  any  metallic  conductor  through  which  a  current  of 
electricity  is  flowing,  the  current-strength  is  directly  pro- 
portional to  the  difference  of  potential  existing  between 
the  ends  of  the  conductor. 

If  the  difference  of  potential  between  the  ends  of  the 
conductor  be  represented  by  (V^  —  F2)  and  the  current- 
strength  by  C,  the  law  can  be  expressed  as  follows: 

~^ — 2  =  K(at  constant  temperature), 


where  K  is  a  constant  depending  only  on  the  conductor. 

For  conductors  of  similar  shape  but  of  different  materials 
the  values  of  K  are  different.  For  wires  composed  of  the 
same  metal  the  following  laws  have  been  determined: 

1.  The  cross-section  of  the  wires  being  uniform  through- 
out, the  value  of  K  is  in  direct  proportion  to  the  length  of 
the  wires. 

2.  The  length  of  the  wires  being  the  same,  the  values  of 
K  are  inversely  proportional  to  the  cross-section  of  the  wires. 

These  two  propositions  apply  only  when  the  temperature 
is  the  same  in  all  cases.  The  value  of  K  for  a  given  metal 
is  found  to  increase  with  increasing  temperature. 

22 


OHM'S  LAW.  23 

It  is  evident  from  the  above  that  if,  with  a  given  differ- 
ence of  electrical  potential,  the  length  of  the  wire  is  in- 
creased, the  current-strength  will  be  diminished.  It  is  there- 
fore customary  to  speak  of  the  constant  K,  which  has  been 
defined  above,  as  the  resistance  of  the  wire  or  body  in 
question,  since  by  increasing  the  length,  or  diminishing  the 
cross-section,  of  the  wire  a  certain  opposition  appears  to  be 
offered  to  the  passage  of  electricity. 

On  the  basis  of  this  conception  it  is  allowable  to  express 
the  law  in  the  following  manner : 

Difference  of  potential 
Current-strength 

and  representing  the  resistance  by  R, 

T/-2  =  #. 

The  unit  employed  in  practice  for  measuring  differences 
of  potential  is  called  the  volt.  If  the  difference  of  potential 
between  the  ends  of  a  conductor  is  equal  to  one  volt  and 
the  current-strength  through  the  conductor  is  equal  to  one 
ampere,  the  value  of  K  in  the  above  equation  would  be 
unity.  Under  these  conditions  the  conductor  is  said  to  have 
unit  resistance,  and  this  unit  has  been  named  the  ohm.  The 
ohm  may  be  defined  as  the  resistance  of  a  conductor,  which 
with  a  difference  of  potential  between  its  ends  of  one  volt, 
permits  of  the  passage  of  a  current  having  a  strength  of  one 
ampere. 

A  resistance  equal  to  the  unit  just  mentioned  is  possessed 
by  a  column  of  mercury,  at  0°  Centigrade,  106.3  centimeters 
in  length  and  with  a  uniform  cross-section  of  one  square 
millimeter. 


24  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

The  mathematical  expression 

Difference  of  potential  in  volts 
Resistance  in  ohms  =  7^ —  ,    . — 

Current-strength  in  amperes 

is  of  great  practical  value,  since  it  makes  it  possible  to  de- 
termine any  one  of  the  three  factors  when  the  other  two  are 
given. 

The  specific  resistance  of  any  substance  is  the  number 
expressing  the  ratio  between  the  resistance  of  a  cylinder  of 
the  substance  in  question  and  the  resistance  of  a  similar 
cylinder  of  some  standard  substance  taken  as  unity.  The 
standard  substance  chosen  is  mercury  at  0°  Centigrade. 

The  form  of  the  equation  given  above  can  be  changed  to 


and  since  R  is  a  constant,  the  value  of  -5  must  be  a  constant 

K 

also.     This  factor,  -p-,  can  be  represented  by  L  and  is  called 

the  conductivity  or  conductance. 

The  equation  may  then  be  written 


If  the  current-strength  is  measured  in  amperes  and  the 
difference  of  potential  in  volts,  the  value  of  the  conductivity 
will  be  obtained  in  reciprocal  ohms.  To  this  unit  the  name 
mho  has  been  given. 


OHM'S    LAW. 


25 


TABLE  OF  THE   SPECIFIC   RESISTANCE  AND  CONDUCTIVITY 
OF  SOME  SUBSTANCES  AT  18°. 


Resistance  in 

Ohms  of  a  Column 

1  Meter  Long  and  1  sq. 

nun.  in  Cross-section. 

Silver 0.016 

Copper      0.0172 

Gold 0.023 

Zinc 0.063 

Iron  0 . 09  to  0 


15 


Platinum 

Lead    . 

Mercury 

Gas  carbon  (about) 


0.14 

0.21 

1.016 

50 


Brass  .  ...     0.07  to  0.09 


German  silver 


0.16  to  0.4 


Conductivity  Referred 
to  Mercury  at  0°. 

59 
55 
41 
15 

6  to  10 

6.5 

4.6 

0.984 

0.02 

10  to  14 

2. 4  to  6 


(Wiedemann   and   Ebert,  Physikalisches    Praktikum,  Braunschweig,. 
1893.) 


CHAPTER  VII. 
MIGRATION  OF  THE  IONS. 

IF  the  solution  of  an  electrolyte  be  submitted  to  elec- 
trolysis, a  movement  of  the  ions  towards  the  electrodes 
and  a  discharge  of  the  ions  at  the  electrodes  will  take 
place.  Equivalent  quantities  of  positive  -and  negative  elec- 
tricity must  disappear  simultaneously  from  the  anode  and 
cathode  respectively,*  and  chemically  equivalent  quantities 
•of  anions  and  cathions  must  be  discharged.  This  must  be 
the  case,  since  if  it  were  not,  the  electrostatic  equilibrium 
of  the  system  would  be  disturbed,  and  under  the  conditions 
this  is  not  possible.  Thus,  for  example,  if  an  excess  of  anions 
separated  at  the  anode  the  solution  would  become  as  a 
whole  charged  negatively,  and  this  condition  would  retard 
the  separation  of  more  anions  and  promote  the  discharge  of 
more  cathions  until  equilibrium  was  again  established. 

If  the  movement  of  electricity  through  the  wires  connect- 
ing the  electrodes  with  the  source  of  current  be  considered 
it  will  be  obvious  that  the  simplest  conception  of  this  is  that 
equal  quantities  of  positive  and  negative  electricity  flow  in 
opposite  directions  through  the  metallic  circuit.  For  this 
and  for  other  reasons  this  is  the  general  conception  of  the 
character  of  current  through  metallic  conductors. 

The  movement  of  the  ions  toward  the  electrodes  during 
electrolysis  is  called  the  migration  of  the  ions.  The  effect 

*  Faraday's  law. 

26 


MIGRATION    OF    THE    IONS. 


27 


of  this  migration  on  the  concentration  of  the  solution  can 
be  best  understood  by  the  use  of  the  following  diagram. 

Let  A  and  B  be  the  positive  and  negative  electrodes 
between  which  is  a  solution  of  a  binary  electrolyte  (an  elec- 
trolyte the  molecules  of  which  dissociate  into  two  univalent 
ions),  and  let  C  be  a  cross-section  of  the  solution  equidistant 
from  A  and  B.  Let  the  form  of  the  vessel  be  such  that  the 


0000 
6 


00© 
c 


©©©©©©b©0©0© 

©©©©©©!©©©©©© 


00©   ©I©   ©  ©   © 

©©©©!©©©© 


©  0-0  ©:0©0©© 
©  ©  ©  ©!©©  ©  ©0 


©  ©  ©  © 


©0© 


FIG.  1. 


volume  of  the  solutions  in  AC  and  EC  are  equal,  and  for  con- 
venience let  it  be  assumed  that  the  quantity  of  the  electro- 
lyte contained  in  each  compartment  is  equal  to  six  mole- 
cules. If  the  velocities  of  migration  of  the  cathion  and  anion 
are  the  same,  then  in  a  given  interval  of  time  if  two  anions 
pass  through  C  from  left  to  right,  two  cathions  will  pass 
through  C  in  the  opposite  direction.  The  state  of  the  solu- 
tion after  this  has  taken  place  is  shown  at  b.  In  this  figure 
the  ions  to  the  left  of  the  anode  and  to  the  right  of  the  cathode 
have  been  neutralised  at  the  electrodes  and  have  passed  out 
from  the  solution. 

It  is  evident  that  under  these  conditions  the  following 
changes  have  taken  place: 

Two  positive  ions  have  passed  into  CB  from  AC  and  four 


28  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

positive  ions  have  been  neutralised  at  the  cathode.  Two 
negative  ions  have  passed  from  CB  into  AC  and  four  nega- 
tive ions  have  been  neutralised  at  the  anode.  Four  equiv- 
alents of  electricity  have  passed  each  of  the  electrodes,  and 
four  equivalents  (two  positive  and  two  negative)  have  also 
passed  the  cross-section  C.  The  current-strength  through 
all  cross-sections  of  the  solution  has  been  the  same.  Equal 
quantities  of  the  electrolyte  remain  on  both  sides  of  the 
section  C. 

If  the  velocities  of  migration  of  the  anion  and  cathion  are 
not  the  same,  a  different  condition  of  affairs  will  be  recog- 
nised. Let  it  be  assumed  that  the  velocity  of  migration  of 
the  cathion  is  twice  that  of  the  anion  (Fig.  1,  c).  Then  in 
a  given  interval  of  time  if  two  cathions  pass  through  C  from 
left  to  right,  only  one  anion  will  have  passed  in  the  opposite 
direction.  Three  cathions  and  three  anions  have  been 
neutralised  at  the  electrodes  and  three  equivalents  of  elec- 
tricity have  disappeared  from  each  of  the  latter.  Three 
equivalents  of  electricity  have  passed  the  cross-section  C, 
two  positive  and  one  negative,.  The  current-strength  through 
all  cross-sections  has  been  the  same. 

The  most  important  result  which  has  been  produced  is 
the  change  in  the  relative  concentration  of  the  solution.  In 
AC  there  are  now  four  molecules  of  the  electrolyte,  while  in 
CB  there  are  five  molecules. 

In  general  when  the  velocities  of  migration  of  the  ions 
are  different  one  of  the  results  of  electrolysis  is  a  change  in. 
the  relative  concentration  of  the  solutions  about  the  cathode 
and  the  anode. 

The  effect  on  the  solution  in  AC  has  been  to  decrease 
the  amount  of  material  contained  in  it  by  two  molecules, 
while  from  CB  a  quantity  equal  to  one  molecule  has  been 
removed.  The  following  equation  can  be  shown  to  express 


MIGRATION   OF   THE    IONS.  29 

the  conditions,  both  for  the  case  mentioned  as  well  as  for 
all  others: 

Velocity  of  migration  of  cathion  _  Loss  at  anode 
Velocity  of  migration  of  anion      Loss  at  cathode"' 

The  terms  ' '  loss  at  anode ' '  and  l l  loss  at  cathode ' '  must 
be  understood  to  mean  the  decrease  in  concentration  of  the 
solutions  in  the  neighborhood  of  the  respective  electrodes. 

If  the  quantity  of  electricity  supplied  to  the  electrodes 
has  been  96,540  coulombs,  and  further,  if  that  portion 
of  a  gram-equivalent  of  the  cathion  which  has  migrated 
through  any  cross-section  of  the  solution  from  the  region 
of  the  anode  toward  the  cathode  be  represented  by  n, 
then  1  —  n  will  represent  that  portion  of  a  gram-equiva- 
lent of  the  anion  which  has  moved  from  the  region  of  the 
cathode  toward  the  anode.  Representing  the  velocities  of 
migration  of  the  cathion  and  anion  by  u  and  v  respectively, 
we  obtain  the  following  equation: 

u        n          Loss  at  anode 
v      l—n     Loss  at  cathode' 

The  quantities  n  and  l—n  are  called  the  shares  of  transport 
or  transport  numbers  of  the  cathion  and  anion. 

In  the  following  table  *  the  relative  velocities  of  migra- 
tion of  a  number  of  cathions  and  anions  are  given : 
(0 . 1  Normal  solutions.) 


K 

=  55.8 

Cl 

=  56.5 

Na 

=  35.0 

I 

-  57.3 

Li 

=  26.1 

N03 

-  51.4 

NH4 

=  54.8 

iS04 

41.9 

Ag 

43.3 

iC02 

=  38 

H 

=  296 

OH 

=  157 

*  Kohlrausch  and  Holborn,  Leitvermogen  der  Elektrolyte. 


CHAPTER  VIII. 
THE  CONDUCTIVITY  OF  SOLUTIONS. 

THE  unit  taken  for  the  conductivity  of  solutions  is  the 
conductivity  of  a  body,  a  column  of  which  one  centimeter 
long  and  one  square-centimeter  in  cross-section  has  the  re- 
sistance of  one  ohm.  The  best  conducting  aqueous  solutions 
of  acids,  at  about  40°,  have  such  a  conductivity. 

Formerly  the  electric  conductivity  was  almost  without 
exception  referred  to  mercury  at  0°.  Since  a  centimeter-cube 
of  mercury  at  0°  has  a  resistance  of  TOT  TO-  ohm,  the  present 
unit  is  10,630  times  greater  than  the  former  one.* 

Since  the  passage  of  electricity  through  the  solution  of 
an  electrolyte  is  primarily  dependent  on  the  ions  which  are 
between  the  electrodes,  and  since  the  greater  the  number 
of  the  ions  the  greater  the  conductivity,  this  factor  in  the 
case  of  solutions  is  of  more  immediate  significance  than  the 
resistance. 

The  simplest  method  of  expressing  the  conductivities 
of  solutions  is  in  terms  of  the  unit  already  denned,  but  since 
it  is  for  many  reasons  desirable  to  compare  solutions  con- 
taining equal  numbers  of  molecules,  the  conductivities  are 
often  expressed  in  terms  of  the  ratio  of  the  conductivity  to 
the  equivalent  or  molecular  concentration. 

Let  a  gram-molecule  of  some  salt,  say  sodium  chloride, 
be  dissolved  in  sufficient  water  to  make  the  total  volume  of 
the  final  solution  equal  to  1000  cc.  and  let  this  solution  be 


*  Kohlrausch  and  Holborn,   Leitvermogen  der  Elektrolyte,    Leipzig,. 
1898. 

30 


THE    CONDUCTIVITY    OF    SOLUTIONS. 


31 


" 


introduced  into  a  rectangular  vessel  having  two  parallel  con- 
ducting walls,  A  and  B  (Fig.  2) ,  which  are  separated  by  a  dis- 
tance of  one  centimeter,  and  let  the  two  other 
sides  and  bottom  be  of  non-conducting  ma- 
terial.     If  the  conductivity   of   the   solution 
between  the  two  electrodes  be  now  measured 
it  will  be  exactly  1000  times  that  of  a  cube 
with   an   edge    of    one   centimeter,   i.e.,  1000 
times  as  great  as  the  conductivity  of  the  given 
normal  solution  expressed  in  the  adopted  units. 

The  equivalent  conductivity  of  the  solution 
may  be  denned  as  the  conductivity  divided 
by  the  equivalent  concentration  (the  concen- 
tration measured  according  to  the  gram-equiv- 
alents of  the  dissolved  substance  in  1  cc.  of  the 
solution). 

If  the  solution  in  the  vessel  be  now  diluted 
to  2000  cc.  and  the  conductivity  of  the  solution 
between  the  electrodes  again  measured,  the 
value  obtained  will  be  2000  times  that  of  a 
cube  of  the  solution  with  an  edge  of  one  cent- 
imeter. 

Since  the  quantity  of  sodium  chloride  be- 
tween the  two  electrodes  is  exactly  the  same 
as  that  in  the  first  measurement  (one  gram- 
molecule)  and  the  distance  between  the 
electrodes  is  unaltered,  any  change  in  the  observed  conduc- 
tivities would  be  due  entirely  to  a  change  in  the  state  of  the 
dissolved  electrolyte.  Since  the  degree  of  dissociation  in- 
creases with  increased  dilution  the  more  dilute  solution 
would  contain  the  greater  number  of  ions,  and  the  con- 
ductivity would  be,  and  in  practice  is  found  to  be,  greater 
in  the  second  case  than  in  the  first. 

If  the  vessel  were  of  sufficient  capacity  so  that  the  -solu- 


FIG.  2. 


32  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

tion  could  be  diluted  until  all  of  the  dissolved  electrolyte 
was  in  the  form  of  ions,  then  the  maximum  value  for  the 
conductivity  would  be  obtained.  There  would  then  be  one 
gram-equivalent  (23  grams)  of  sodium  ions  and  one  gram- 
equivalent  (35.5  grams)  of  chlorine  ions  between  the  two 
electrodes. 

A  description  of  the  apparatus  and  method  used  in  actual 
practice  for  measuring  the  conductivity  of  solutions  is  be- 
yond the  scope  of  the  present  work.  It  may  be  stated  that 
the  general  principle  depends  upon  the  measurement  of 
the  resistance  of  the  solution  which  is  contained  in  a  suitable 
vessel  provided  with  platinum  electrodes.  In  calculating  the 
conductivity  of  the  solutions  from  the  values  thus  obtained 
use  is  made  of  the  so-called  "resistance  capacity"  of  the 
vessel,  a  factor  which  depends  upon  the  shape  and  size  of, 
and  the  distance  between,  the  electrodes.  Since  direct 
currents  would  produce  a  decomposition  of  the  solution  only 
alternating  currents  of  high  frequency  and  low  current- 
strength  are  employed  in  measuring  the  conductivity. 

Since  strong  acids,  strong  bases  and  salts  are  completely 
dissociated  in  solutions  which  are  not  so  dilute  as  to  make 
accurate  measurements  of  their  conductivities  impossible, 
the  equivalent  conductivities  of  a  large  number  of  these  com- 
pounds have  been  determined  under  all  conditions  of  dilution. 
If  the  equivalent  conductivity  of  an  electrolyte  at  any  given 
concentration  be  divided  by  the  equivalent  conductivity  of 
a  solution  in  which  the  dissociation  is  complete  the  quotient 
will  be  the  degree  of  dissociation  of  the  dissolved  substance 
at  the  given  concentration  (see  p.  12). 

As  a  general  summary  of  the  theory  of  electrolytic  con- 
ductivity it  may  be  said  that  since  the  passage  of  electricity 
through  an  electrolyte  is  always  accompanied  by  the  transfer 
of  matter,  the  power  which  a  solution  has  for  conducting  the 
electric  current  must  depend  directly  on  the  nature  of  the 


THE    CONDUCTIVITY    OF    SOLUTIONS. 


33 


substances  which  are  in  the  solution.  The  ions  alone  are 
the  bearers  of  electric  charges,  the  undissociated  molecules 
taking  no  part  in  the  transportation  of  electricity.  The  con- 
ductivity of  a  solution  depends,  therefore,  upon  the  number 
of  ions  which  it  contains  and  the  nature  of  the  ions  themselves. 
The  speed  at  which  the  electricity  is  transported  depends 
upon  the  velocity  of  migration  of  the  ions.  The  general 
relations  are  expressed  in  the  law  known  as  Kohlrausch's 
law,  which  states  that: 

The  equivalent  conductivity  of  an  electrolyte  is  equal 
to  the  sum  of  two  values,  one  of  which  depends  solely  upon 
the  cathion,  the  other  upon  the  anion. 

The  effect  of  a  rise  in  temperature  is,  in  general,  to  in- 
crease the  conductivity  of  solutions. 

The  conductivity  of  solutions  is  governed  by  Ohm 's  law. 
CONDUCTIVITY*  OF  AQUEOUS  SOLUTIONS  (18°  C.). 


Concentration  expressed 
in  per  cent,  of  dissolved 
substance. 

Substance. 

Conductivity  in  terms 
of  unit  defined  p. 

10 

HC1 

0.63 

10 

HBr 

0.35 

12.4 

HNO3 

0.54 

10 

H2S04 

0.39 

9.8 

Acetic  acid 

0.0015 

10 

Tartaric  acid 

0.0081 

7 

Oxalic  acid 

0.078 

10 

H3P04 

0.056 

8.4 

KOH 

0.27 

10 

NaOH 

0.31 

8 
6.5 

KCN 

0.001 
0.10 

10 
10 

AgN03 

Cu(N03)2 

0.047 
0.063 

10 

K2S04 

0.086 

10 

Na2SO4 

0.068 

10 
10 

(NH4)2S04 
ZnSO4 

0.101 
0.032 

10 

CuSO4 

0.032 

13 
10 

10 

NiSO4 
K2C204 
KH2PO4 

0.045 
0.091 
0.040 

*  Kohlrausch  and  Holborn,  Leitvermogen  der  Elektrolyte. 


CHAPTER   IX. 
ELECTROLYSIS. 

SINCE  an  electrolyte  is  already  dissociated  into  its  ions 
in  a  solution,  the  action  of  the  current  during  electrolysis  is 
not  the  decomposition  of  the  dissolved  substance  but  is  con- 
fined to  the  transportation  and  discharge  of  the  ions  at  the 
electrodes.  When  the  electrical  charges  of  the  ions  are 
neutralised  at  the  electrodes  these  particles  instantly  acquire 
the  properties  of  atoms  or  atomic  radicals. 

When  a  solution  of  potassium  sulphate  is  electrolysed 
between  platinum  electrodes,  hydrogen  is  liberated  at  the 
cathode  and  oxygen  at  the  anode.  Chemically  equivalent 
quantities  of  the  two  gases  appear,  i.e.  two  volumes  of 
hydrogen  to  every  one  volume  of  oxygen.  It  might  be  as- 
sumed from  this  that  the  ions  in  the  solution  are  hydrogen 
and  oxygen ;  but  if  after  the  electrolysis  has  proceeded  some 
time  the  solutions  about  the  electrodes  are  examined  it  is 
found  that  the  solution  about  the  cathode  contains  potas- 
sium hydroxide,  while  that  about  the  anode  contains  sul- 
phuric acid,  and  further,  the  quantities  (measured  in  gram- 
equivalents)  of  hydrogen  and  oxygen  set  free  are  exactly 
equivalent  to  the  quantities  of  the  acid  and  base  in  the  solu- 
tion, measured  in  the  same  units. 

It  was  formerly  customary  to  speak  of  the  base  and  acid 

as  resulting  from  the  secondary  action  of  the  atomic  potas- 

34 


ELECTROLYSIS.  35 

slum  and  S04  on  the  water  of  the  solution,  and  to  express 
the  complete  reaction  by  the  following  equations: 

K2S04  (solid)  =  K  +  K  +  SO4  (solution). 
On  electrolsis  — 


+  SO4  (ions)=K2  +  S04  (atomic), 
2K2  +  2H2O  =  2KOH+2H2  (gaseous), 
2S04  +  2H20  =  2H2S04  +  02  (gaseous)  . 

This  view  of  the  behavior  of  the  ions  at  the  electrodes, 
while  it  has  certain  advantages  as  a  method  of  expressing 
the  reactions,  has  the  disadvantage  that  it  does  not  presum- 
ably give  an  accurate  idea  of  the  theoretical  course  of  the 
electrolysis.  It  is  therefore  preferable  to  assume  that  the 
hydrogen  and  oxygen  which  appear  in  the  above  electrolysis 
are  due  to  the  primary  dissociation  of  the  water,  and  the 
course  of  the  electrolysis  can  then  be  assumed  to  '  be  the 
following: 

+ 

Under  the  influence  of  the  charged  electrodes  the  K  and 

S04  ions  in  the  solution  migrate  to  the  cathode  and  anode, 
and  when  they  arrive  there,  those  ions  whose  separation  is 
attended  with  the  least  expenditure  of  energy  will  separate 
on  the  electrodes.  Thus,  for  example,  in  the  region  of  the 
cathode  there  will  be  potassium  ions  and  hydrogen  ions 
(the  latter  from  the  primary  dissociation  of  the  water). 
When  an  excess  of  positive  electricity  is  present  in  this  por- 
tion of  the  solution,  and  a  discharge  of  cathions  takes  place, 
the  hydrogen  ions  will  be  the  first  to  discharge,  since  the 
separation  of  these  is  attended  with  a  smaller  expenditure 
of  energy  than  would  be  required  for  the  separation  of  the 
potassium  ions.  The  OH  residue  of  the  water  and  the 
potassium  ions  constitute  the  potassium  hydroxide  found 


36  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

in  this  portion  of  the  solution.  The  action  at  the  anode 
is  explained  in  an  analogous  manner.  It  must  be  remem- 
bered, however,  that  the  degree  of  dissociation  of  the  sol- 
vent water  is  extremely  small,  so  small  in  fact  as  to  con- 
tribute to  the  conductivity  of  the  solution  to  a  scarcely 
appreciable  extent.  But  this  does  not  limit  the  actual 
quantity  of  hydrogen  and  oxygen  ions  which  in  a  given 
time  can  be  supplied  from  this  source  to  the  electrodes 
since  no  sooner  are  the  ions  present  removed  than  a  new 
supply  is  instantly  furnished  by  the  dissociation  of  a  fresh 
quantity  of  water. 

If  the  electrolysis  of  a  potassium  sulphate  solution  be 
conducted  with  a  mercury  cathode  and  the  current  be  suit- 
ably regulated,  hydrogen  will  not  appear.  Instead  of  this 
an  amalgam  of  potassium  with  the  mercury  of  the  cathode 
will  be  obtained.  This  is  explained  by  the  fact  that  the 
potassium  ions,  since  they  can  immediately  enter  into  com- 
bination with  the  mercury,  separate  more  readily  than  the 
hydrogen  ions  under  these  conditions. 

If  a  solution  of  chromic  acid,  H2Cr04,  containing  sul- 
phuric acid  is  electrolysed,  chromic  sulphate,  Cr2S04,  is 
formed  at  the  cathode  and  no  hydrogen  appears.  In  this 
case  the  reaction  can  be  explained  by  assuming  that  hydro- 
gen is  first  set  free,  and  then  as  a  secondary  reaction  reduces 
the  chromic  acid  to  the  divalent  chromium  compound;  but 
it  is  more  logical  to  assume  that  under  the  electrical  condi- 
tions existing  at  the  cathode  the  Cr04  ion  changes  to  three 
+ 

Cr  ions,  since  the  electrical  equivalence  of  the  change  is  the 
same  and  there  is  no  actual  evidence  that  the  hydrogen  is 
even  temporarily  deposited  on  the  electrode.  As  a  method 
of  expression,  however,  the  former  assumption  is  the  more 
convenient  and  for  this  reason  will  be  followed.  The  reac- 


ELECTROLYSIS.  37 

tion  at  the  cathode  can  therefore  be  expressed  by  the  fol- 
lowing equations: 

H2Cr04  =  H2  +  CrO4, 

2CrO4  +  5H2  +  3H2S04  =  Cr2(S04)3+8H20. 


The  most  important  factor  in  electrolytic  experiments  , 
next  to  the  actual  composition  of  the  solution,  is  the  ratio  of 
the  current-strength  to  the  surface-area  of  the  electrodes. 
It  is  obvious  that  the  reactions  which  take  place  at  an  elec- 
trode must  be  greatly  influenced  by  the  relative  number  of 
ions  which  are  neutralised  per  unit  area  of  electrode  surface. 
The  ratio  mentioned  above  is  known  as  the  current-density, 
and  as  unit  a  current-strength  of  one  ampere  for  100  square 
centimeters  of  electrode  surface  has  been  chosen.  This  unit 
is  known  as  the  normal  density,  and  current-densities  ex- 
pressed in  terms  of  this  unit  are  denoted  by  the  symbol  ND100. 
The  expression  ND100  =  1.2  amperes,  denotes  that  for  every 
100  square  centimeters  of  the  given  electrode  a  current  of 
1.2  amperes  is  passing  through  the  circuit. 

Although  the  current-strength  is  the  same  through  every 
cross-section  of  the  circuit,  the  current-densities  at  the  cathode 
and  anode  have  the  same  value  only  when  the  surfaces  of 
these  in  contact  with  the  solution  are  exactly  equal. 

When  a  solution  of  copper  sulphate  is  electrolysed  and 
the  current-density  at  the  cathode  is  properly  regulated, 
only  metallic  copper  is  deposited;  but  when  the  current- 
density  is  too  high  the  metallic  copper  is  mixed  with  copper 
oxide,  and  hydrogen  is  also  set  free.  Similarly,  in  the  case  of 
cupric  chloride  solutions,  copper  or  cuprous  chloride  appears 
at  the  cathode  ;  while  variations  in  the  current-density  at  the 
anode  in  the  electrolysis  of  sulphuric  acid  solutions  result  in 
the  appearance  of  oxygen,  ozone,  hydrogen  peroxide,  or 
persulphuric  acid.  A  classical  illustration  of  the  effect  of 


38  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  current-density,  described  by  Bunsen,  is  furnished  by 
the  electrolysis  of  a  solution  of  chromium  chloride  where 
either  hydrogen,  chromic  oxide,  chromous  oxide,  or  metallic 
chromium  can  be  obtained.  The  concentration  and  tem- 
perature of  the  solution  is  also  an  important  factor  in  these 
reactions. 

In  the  electrolysis  of  hydrochloric  acid  the  chlorine  set 
free  at  the  anode  reacts  with  water,  forming  hypochlorous 
acid,  chloric  acid,  perchloric  acid,  etc.  Similar  secondary 
reactions  are  observed  in  the  electrolysis  of  chlorides.  If  a 
solution  of  ammonium  chloride,  for  example,  is  submitted 
to  electrolysis  the  nascent  chlorine  acts  on  the  undecom- 
posed  salt,  with  the  production,  among  other  substances, 
of  nitrogen  or  nitrogen  chloride.  Halogen  salts  of  the  alka- 
line earths  show  similar  phenomena. 

Nitric  acid,  on  electrolysis,  gives  in  the  first  place 

8HN03  =  4H2  (cathion)+8N03  (anion). 
The  latter  then  splits  up  further : 

4N206  =  4N205  +  202  (anion) . 

The  oxygen  is  given  off,  while  the  anhydride  forms  nitric 
acid  again  with  water: 

4N205+4H20  =  8HN03. 

The  hydrogen,  on  the  contrary,  which  appears  as  cathion, 
is  not  set  free  but  acts  reducingly  on  the  nitric  acid  present: 

4H2  +  HN03  -  NH3  +  3H20. 

In  the  presence  of  sulphuric  acid,  or  a  sulphate,  this  de- 
composition is  complete,  the  final  product  being  ammonium 
sulphate. 

In  the  electrolysis  of  organic  compounds  the  ions  as  they 


ELECTROLYSIS.  39 

are  released  from  the  solution  at  the  electrodes  may  enter 
into  secondary  reactions  in  a  manner  analogous  to  that 
described  in  the  case  of  inorganic  compounds. 

Thus  potassium  acetate  might  be  expected  to  yield  potas- 
sium (potassium  hydroxide)  and  acetic  acid  as  final  prod- 
ucts, but  instead  of  the  latter  the  acetic  ion  splits  up  into 
carbon  dioxide  and  ethane,  according  to  the  equation: 


CH3COO 
CH3COO 


> 


and  ethylene  may  also  be  formed  from  the  oxidation  of 
ethane  at  the  anode. 

Potassium  valerate  yields,  in  addition  to  valeric  acid, 
carbon  dioxide  and  octane;  and  by  continued  electrolysis 
the  latter  is  oxidised  into  isobutylene  and  water.  In  the 
electrolysis  of  sodium  succinate  among  the  products  formed 
are  ethylene  and  carbon  dioxide;  potassium  lactate  breaks 
up  into  carbon  dioxide  and  acetaldehyde  ;  potassium  tar- 
trate  gives  carbon  dioxide,  carbon  monoxide,  oxygen,  for- 
mic aldehyde,  and  formic  acid;  and  potassium  cyanide  is 
converted  into  potassium  cyanate. 

The  electrolysis  of  warm  solutions  of  oxalic  acid  lead 
to  the  complete  decomposition  of  the  acid  according  to  the 
equation 

C2H2O4  =  2C02(anode)  +H2(cathode). 

In  cold  solutions  carbon  monoxide,  as  well  as  carbon  dioxide, 
appears  at  the  positive  electrode. 

When  potassium  oxalate  is  subjected  to  electrolysis 
the  principal  reactions  are: 

'K2C204  =  2C02(anode)+K2(cathode), 


=  2KHC0. 


40  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

When  ammonium  oxalate  is  used  the  decomposed  solu- 
tion contains  hydrogen  ammonium  carbonate;  a  part  of  the 
latter  splits  up  into  ammonia  and  carbon  dioxide. 

In  the  electrolysis  of  double  oxalates,  e.g.  zinc  ammo- 
nium oxalate,  decomposition  takes  place  as  follows:  Zinc 
oxalate  breaks  up  into  zinc  and  carbon  dioxide,  and  ammo- 
nium oxalate  into  ammonia  and  carbon  dioxide.  The 
carbon  dioxide  which  separates  at  the  anode  combines  with 
the  ammonia  in  the  solution  to  form  hydrogen  ammonium 
carbonate  as  explained  above. 

The  simplest  conditions  are  met  in  electrolysis  in  those 
cases  where  the  ions  in  the  solution  on  reaching  the  elec- 
trodes are  immediately  deposited  and  thus  removed  from 
the  solution.  Such  is  the  case  for  example  in  the  electrolysis 
of  a  solution  of  stannous  chloride  between  a  platinum  cathode 
and  a  silver  anode:  the  stannous  ions  are  deposited  on  the 
cathode  as  metallic  tin,  and  the  chlorine  ions  at  the  moment 
of  electrical  neutralisation  combine  with  the  silver  of  the 
anode  to  form  an  insoluble  coating  of  chloride  of  silver. 

So  also  in  the  case  of  the  electrolysis  of  a  copper  sul- 
phate solution  between  copper  electrodes,  metallic  copper 
is  deposited  at  the  cathode,  while  an  exactly  equal  quantity 
of  copper  passes  into  solution  at  the  anode. 

For  the  continuous  electrolysis  of  any  solution  between 
electrodes  which  are  not  attacked,  a  certain  minimum  poten- 
tial difference  is  essential.  This  factor,  which  is  of  great  im- 
portance in  electrochemical  analysis,  is  called  the  decomposi- 
tion potential,  and  by  it  is  denoted  that  difference  of  potential 
at  which  the  electric  current  is  just  able  to  transform  ions  into 
atoms  at  the  electrodes.*  Le  Blanc,  who  measured  the  de- 


*  The  significance  of  this  factor  will  be  discussed  later  in  the  chapter 
on  Polarisation 


ELECTROLYSIS. 


41 


composition  potential  of  a  large  number  of  solutions,  deter- 
mined the  following  values: 


SALTS. 


Normal  (Molecular)  Solutions. 
ZnSO4 2.35 volts.     Cd(XO3)2 1 .98 volts. 


ZnBr2 1.80 

NiSO4 2.09 

NiCl2 1.85 

Pb(NO3)2 1.52 

AgNO, 0.70 


CdSO4 2.03 

CdCl2 1.88 

CoSO4 1.92 

CoCl2 1.78 


Sulphuric  acid 

Nitric  acid 

Phosphoric  acid 

Monochloracetic  acid. 
Dichloracetic  acid.  .  .  . 

Malonic  acid 

Perchloric  acid 

Dextrotartaric  acid.  . 


ACIDS. 
.  67  volts.     Pyroracemic  acid 


.69 
.70 
.72 
.66 
.69 
.65 
.62 


1.57  volts. 

Trichloracetic  acid 1.51 

Hydrochloric  acid 1 .31 

Hydrazoic  acid 1 . 29 

Oxalic  acid 0.95 

Hydrobromic  acid 0.94 

Hydriodic  acid 0 . 52 


Sodium  hydroxide 1 . 69  volts. 

Potassium  hydroxide.  . .    1 . 67     ' ' 
Ammonium  hydroxide  .    1 . 74     " 


0 .  4X.  Methylamine 1 . 75  volts. 

0 .  2N.  Diethylamine 1 . 68     " 

0.8X.  Tetramethyl  am- 
monium hydroxide  1.74     " 

The  sulphates  and  nitrates  of  the  alkalies  and  alkaline 
earths  have  all  nearly  the  same  decomposition  potential, 
namely,  about  2.20  volts. 


CHAPTER   X. 
ELECTROMOTIVE   FORCE. 

IF  a  strip  of  metal,  zinc  for  example,  be  dipped  into  a 
solution  of  a  zinc  salt,  the  zinc  ions  present  in  the  solution 
will  have  a  tendency  to  discharge  their  electricity  upon  the 
zinc  and  to  pass  over  into  the  atomic  condition.  This  ten- 
dency may  be  considered  as  a  pressure  striving  to  force  the 
ions  from  the  solution  toward  the  metal,  and  is  known  as 
the  osmotic  pressure  of  the  ions.*  The  metallic  zinc,  however, 
may  be  considered  to  exert  a  pressure  in  the  opposite  direc- 
tion, which  is  due  to  the  tendency  of  the  zinc  atoms  to  pass 
into  the  solution  and  assume  the  condition  of  ions.  This 
pressure  is  known  as  the  electrolytic  solution  pressure  of  the 
zinc.  Since  the  ions  are  bearers  of  electric  charges  it  is  evi- 
dent that  the  simultaneous  action  of  these  two  pressure-forces 
is  intimately  connected  with  the  production  of  an  electric 
current,  and  that  as  one  or  the  other  of  these  predominates, 
positive  electricity  will  pass  from  the  region  of  higher 
pressure  to  the  region  of  lower,  and  a  difference  of  potential 
between  the  solution  and  the  metal  will  result.  The  osmotic 
pressure  of  the  ions  and  the  electrolytic  solution  pressure 
of  the  metal  tend  to  cause  currents  in  opposite  directions. 
The  positively  charged  ions  of  the  solution  strive  to  give  up 
their  charges  to  the  exposed  metal  and  to  charge  this  positive ; 
while,  on  the  other  hand,  the  electrolytic  solution  pressure 

*Nernst,  Zeit.  f.  physik.  Chem.,  4,  129  (1889). 

42 


ELECTROMOTIVE   FORCE.  43 

strives  to  force  positive  ions  out  from  the  metal  and  into  the 
solution,  leaving  an  equivalent  negative  charge  on  the  metal 
itself. 

Since  the  operation  of  all  galvanic  elements  depends 
upon  the  difference  of  potential  bet  ween  conductors  of  the 
first  order*  and  solutions  of  electrolytes,  the  hypothesis 
just  enunciated  is  of  great  value  in  offering  an  explanation 
of  the  working  of  these  cells. 

A  very  simple  cell  is  furnished  by  the  combination,  me- 
tallic silver,  silver  nitrate  solution  (concentrated),  silver 
nitrate  solution  (dilute),  metallic  silver.  In  this  combina- 
tion the  metallic  silver  in  contact  with  the  concentrated 
silver  nitrate  solution  is  found  to  be  positively  charged  with 
respect  to  the  silver  electrode  dipping  into  the  dilute  solu- 
tion. When  the  solutions  are  0.1  normal  and  0.01  normal 
respectively,  the  difference  of  potential  is  0.055  volt  (at  18°). 
The  theory  offers  the  following  explanation  of  this  cell:  The 
-electrolytic  solution  pressure  of  the  metallic  silver  is  the 
same  at  both  poles,  but  in  the  concentrated  solution  the 
osmotic  pressure  of  the  silver  ions  is  greater  than  in  the 
dilute  solution.  There  will  therefore  be  a  tendency  for 
silver  ions  to  pass  out  of  the  concentrated  solution  and  to 
deposit  as  metallic  silver  on  the  electrode  in  this  solution, 
and,  simultaneously,  for  silver  atoms  to  pass  out  as  ions 
from  the  silver  electrode  into  the  dilute  solution.  The 
silver  electrode  in  the  concentrated  solution  will  thus  be- 
come positively  charged  with  respect  to  that  in  the,  dilute 
solution.  As  this  process  continues  the  electrostatic  con- 
dition will  become  such  that  the  difference  of  potential 
between  the  solution  and  the  electrodes  will  oppose  the  pre- 
cipitation and  formation  of  any  further  ions,  and  a  state  of 

*  Substances  which  conduct  the  electric  current  without  undergoing 
xjhemical  decomposition,  i.e.  metals,  carbon,  etc. 


44  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

equilibrium  will  be  reached  which  will  depend  upon  the  rela- 
tive concentrations  of  the  two  solutions,  and  will  be  measured 
by  the  difference  of  potential  between  the  two  poles,  as  stated 
above.  If,  however,  the  two  poles  of  the  cell  are  connected 
with  one  another  by  a  metallic  conductor,  so  that  this  electro- 
static difference  of  potential  is  prevented,  the  process  of  pre- 
cipitation and  solution  will  continue  until  both  solutions  are 
of  equal  concentration. 

The  electrolytic  solution  pressures  of  a  number  of  metals 
have  been  calculated  by  Neumann*  and  their  values  are 
given  in  the  following  list : 

Zinc     .      ;  .;,.-".;  =  9.9Xl018    atmospheres. 

Cadmium  .,.      .      .      .     v  =  2.7xl06 

Iron     .      .  .      .   !      .    :.      .  =1.2X104 

Nickel       /  .      .      .      .     .     .  =1.3X10° 

Lead    .     .  .     .     .     .     .     .  =l.lXlQ-3 

Hydrogen  .      .    ;.      .      .      .  =  9.9xlO~4  ". 

Copper      .  .      .      .      .      .....  =4.8X10-20 

Mercury    .  ,      ....      .  =l.lxl0-10          " 

Silver.      .  .      .      ,x.-    .     :  =  2.3xlO-17 

Assuming  the  osmotic  pressure  of  the  ions  in  a  totally  dis- 
sociated normal  solution  to  be  22  atmospheres,  it  is  evident 
from  the  above  list  that  electrodes  of  zinc,  cadmium,  and 
iron  will  always  be  charged  negatively  with  respect  to  normal 
solutions  of  their  ions  with  which  they  are  in  contact,  while 
solutions  of  nickel,  lead,  hydrogen,  copper,  mercury  and 
silver,  *under  similar  conditions,  will  always  be  charged 
positively. 

A  familiar  galvanic  cell  is  the  so-called  Daniell's  ele- 
ment. This  consists  of  a  zinc  electrode  surrounded  by  a 
solution  of  zinc  sulphate,  and  a  copper  sulphate  solution 

*  Zeit.  f.  phys.  Chem.,  14,  229  (1894). 


ELECTROMOTIVE   FORCE.  45 

containing  an  electrode  of  metallic  copper.  If  the  solutions 
are  both  normal,  then  the  electrolytic  solution  pressure  of 
the  zinc  will  greatly  exceed  the  osmotic  pressure  of  the  zinc 
ions  in  the  solution,  and  positive  zinc  ions  will  pass  from  the 
metal  to  the  solution,  leaving  a  negative  charge  on  the  former. 
The  osmotic  pressure  of  the  copper  ions  in  the  copper  sul- 
phate solution  will  exceed  the  electrolytic  solution  pressure 
of  the  copper,  and  accordingly  copper  ions  will  pass  from 
the  solution  to  this  electrode  and  charge  it  positive.  The 
difference  of  potential  thus  produced  will  consist  primarily 
of  the  sum  of  two  differences,  namely,  that  between  the  zinc 
and  the  zinc  sulphate  solution,  and  that  between  the  copper 
sulphate  solution  and  the  copper.  This  total  difference  of 
potential  is  equal  to  1.1  volts. 


TABLE. 

SINGLE   POTENTIAL  DIFFERENCES   OF  METALS 
OF  THEIR  SALTS.* 

IN    NORMAL   SOLUTIONS 

Zinc     

.      +0.51  volt. 

Cadmium       

.      +0.16    " 

Iron     

.      +0.09    " 

Nickel       
Lead    

.      -0.02    " 
-0.10    " 

Hydrogen       .      .      . 
Copper      
Mercury    

.      -0.25    " 
.      -0.59    "     , 
-1.03    " 

Silver  . 

-1.06    " 

The  signs  denote  the  charge  of  the  solution  when  the  po- 
tential of  the  metal  is  placed  at  zero. 

It  is  evident  that  if  the  two  electrodes  (or  poles)  of  the 
cell  are  connected  by  a  metallic  conductor  their  charges  will 
be  removed  and  the  difference  of  potential  between  them  will 

*  Ostwald,  Grundriss  der  Allgemeinen  Chemie,  3  Auf.,  1899,  p.  468. 


46  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

be  diminished.  This  fall  in  potential  between  them  will  be 
proportional  to  the  resistance  of  the  connecting  conductor. 
It  is  evident  also  that  this  difference  of  potential  between 
them  will  have  a  maximum  value  when  they  are  not  exter- 
nally connected.  Since  this  maximum  value  is  the  true 
measure  of  the  electrical  factors  in  the  cell,  it  will  be  given  a 
special  name  and  will  be  called  the  electromotive  force  of  the  cell. 
The  internal  resistance  of  a  cell  is  the  resistance  of  the 
solution,  or  solutions,  which  it  contains  and  the  porous  sepa- 
rating wall,  if  one  is  used  to  separate  the  solutions. 


CHAPTER  XI. 

POLARISATION. 

IF  a  solution  of  zinc  sulphate  is  electrolysed  between 
platinum  electrodes,  metallic  zinc  will  be  deposited  on  the 
cathode  and  oxygen  on  the  anode.  As  soon,  however,  as  the 
metal  and  the  oxygen  have  passed  over  into  the  atomic  con- 
dition, the  electrolytic  solution  pressure  of  each  comes  into 
action  and  operates  to  drive  them  back  into  the  form  of 
ions.  The  electrodes  can  no  longer  be  regarded  as  consisting, 
of  platinum,  but  behave  exactly  as  if  composed  of  zinc  and 
oxygen.  Under  these  conditions  the  electrodes  are  said  to 
have  become  polarised  and  the  phenomenon  is  known  as 
polarisation.  The  combination  may  then  be  considered 
as  a  galvanic  cell  consisting  of  zinc,  sulphuric  acid,  and  oxy- 
gen, with  an  electromotive  force  opposed  to  that  of  the  first 
or  primary  current. 

If  the  primary  current  be  now  interrupted,  and  the  two 
electrodes  are  connected  by  a  metallic  conductor,  a  current 
will  flow  between  them  through  the  conductor  in  a  direction 
opposite  to  that  of  the  original  current.  The  current  thus 
obtained  is  called  the  polarisation  current.  Polarisation  will 
always  take  place  when  the  electrodes  are  composed  of  a 
metal  like  platinum,  which  is  not  attacked  during  the  elec- 
trolysis. 

For  the  continuous  electrolytic  decomposition  of  any 
solution  a  difference  of  potential  must  therefore  be  maintained 

47 


48      QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS. 

between  the  electrodes  greater  than  the  back  electromotive 
force  due  to  polarisation.  Unless  this  is  the  case  no  current 
will  pass  through  the  cell.* 

It  is  evident  that  a  very  close  relation  must  exist  between 
the  electromotive  force  due  to  polarisation  and  the  decom- 
position potential  of  solutions  measured  by  Le  Blanc  (page  41), 
since  these  latter  values  represent  the  difference  of  potential 
between  the  electrodes  necessary  in  order  to  overcome  the 
back  electromotive  force  due  to  polarisation. 

The  current-strength  through  any  circuit  containing  an 
electrolytic  cell  may  be  calculated  from  the  equation: 


R   ' 

where  P  is  the  primary  difference  of  potential,  p  is  the  elec- 
tromotive force  due  to  polarisation,  and  R  is  the  total  resist- 
ance of  the  circuit. 

*  It  has  been  observed  that  very  small  currents  will  flow  through  a  cell 
«ven  when  the  difference  of  potential  between  the  electrodes  is  considerably 
lower  than  the  values  obtained  for  the  decomposition  potential  by  Le 
Blanc  (p.  41).  A  satisfactory  explanation  of  this  phenomenon  is  yet 
to  be  discovered. 


SECTION  II.— DESCRIPTIVE. 


CHAPTER   XII. 
ELECTROCHEMICAL   ANALYSIS. 

THE  'object  of  electrochemical  analysis  is  the  quantitative 
determination  of  certain  elements,  chiefly  metals,  by  the  use 
of  the  electric  current.  For  this  purpose  the  elements,  or 
their  compounds,  are  precipitated  in  an  adherent  form  on 
weighed  electrodes  and  their  quantity  is  determined  from 
the  increase  in  weight  of  the  electrode.  Electrolytic  reac- 
tions are  also  employed  for  separating  an  element  from  one 
or  several  others,  since  the  conditions  favorable  for  the  pre- 
cipitation of  one  element  are  often  such  as  entirely  prevent 
the  precipitation  of  another.  One  of  the  chief  advantages 
of  the  electrochemical  method  is  that  when  the  electrolytic 
process  is  once  started  it  requires  little  or  no  attention  until 
completed.  Another  advantage  lies  in  the  fact  that  many 
separations  of  different  elements  can  be  effected  without 
difficulty  by  this  means  which  are  extremely  troublesome 
when  conducted  by  the  ordinary  gravimetric  methods.  The 
extreme  accuracy  of  many  of  the  electrolytic  determinations 
is  also  a  strong  point  in  their  favor. 

Quantitative  electrochemical  determinations  may  be 
divided  into  two  general  classes  according  to  whether  the 

49 


50  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

determination  of  a  cathion  (metal)  or  an  anion  (halogen  or 
metal  peroxide)  is  involved.  The  metals  are  precipitated  as 
metallic  coatings  on  the  cathode,  and  the  halogens  are  deter- 
mined by  the  employment  of  a  silver  anode  with  which  they 
combine  to  form  silver  halogen  compounds.  The  process  in 
the  separation  of  the  metal  peroxides  is  somewhat  more 
complicated.  Formerly  the  formation  of  lead  and  manga- 
nese peroxides  was  attributed  to  an  oxidation  brought  about 
by  the  electrolytically  generated  oxygen.  The  investigations 
of  Liebenow  *  and  Lob  f  have  made  it  appear  that  lead  per- 
oxide and  manganese  peroxide  ions  are  already  present  in 
the  solutions.  Since  the  peroxides  are  deposited  from  strong 
nitric  acid  solutions,  it  is  necessary  to  assume  that,  through 
the  oxidising  power  of  this  acid,  oxygen  ions  are  formed  in 
the  solutions,  and  that  these  oxygen  ions  combine  with  the 
lead  and  manganese  ions  to  form  peroxide  ions.  Since  in 
the  peroxides  of  the  bivalent  metals  the  two  positive  charges 
of  the  metal  are  combined  with  the  four  negative  charges  of 
the  two  oxygen  ions,  the  resulting  peroxide  ion  possesses  two 
negative  charges  and  consequently  behaves  like  a  bivalent 
anion.  It  is  precipitated  on  the  positive  electrode  in  a 
smooth,  adherent  coating. 

In  electrochemical  analysis  the  composition  of  the  solu- 
tion from  which  a  metal  is  separated  is  of  the  very  greatest 
importance,  since,  in  general,  the  metals  do  not  separate 
from  solutions  of  their  pure  salts  in  a  form  which  is  suitable 
for  quantitative  determination.  For  this  reason  solutions 
of  mixed  electrolytes  are  employed  almost  without  excep- 
tion for  this  purpose,  since  the  addition  of  other  substances 
to  the  solution  of  the  metal  salt  has  a  very  important  influ- 
ence on  the  physical  character  of  the  precipitated  metal.  By 

*  Zeit.  fur  Elektrochemie,  3,  653  (1896-97). 
i&,  3,  100  (1896-97). 


ELECTROCHEMICAL    ANALYSIS.  51 

the  addition  of  suitable  substances  to  the  solution  the  precipi- 
tated metal  can  be  obtained  as  a  smooth,  compact  and  firmly 
adherent  coating  on  the  electrode  in  cases  where  if  a  solution 
of  the  pure  salt  were  used  the  metal  would  be  deposited 
as  a  crystalline  or  amorphous  crust  totally  unsuited  to  accu- 
rate determination.  Substances  are  also  added  to  the  solu- 
tions to  permit  the  complete  precipitation  of  certain  metals. 
Their  action  is  then  to  neutralise  or  counteract  the  effect  of 
deleterious  substances  formed  during  the  process  of  elec- 
trolysis. Thus  by  the  addition  of  ammonium  oxalate  to  a 
solution  of  zinc  sulphate,  the  zinc  can  be  completely  precipi- 
tated, which  would  not  be  the  case  if  the  action  of  the  sul- 
phuric acid  formed  at  the  anode  were  not  neutralised  by 
the  ammonium  oxalate  in  the  solution.  Chemical  substances 
other  than  the  salts  of  the  metals  to  be  determined  are  also 
added  to  solutions  in  order  to  permit  the  separation  of  one 
metal  from  one  or  several  others.  Thus  the  addition  of  nitric 
acid  to  a  solution  containing  the  sulphates  of  copper,  iron,  and 
zinc,  makes  it  possible  to  completely  separate  the  copper  by 
electrolysis  without,  at  the  same  time,  bringing  down  even 
traces  of  the  other  two  metals. 

If  the  position  of  hydrogen  is  represented  by  a  dash,  and 
those  elements  which  separate  more  readily  than  hydrogen 
are  placed  below  this  line,  while  those  which  separate  less 
readily  are  placed  above  it,  the  conditions  existing  in  various 
different  solutions  may  be  represented  as  given  below  :  * 

Cu,  Pt,  As,  Xi,  Co,  Fe,  Zn 

in  potassium  cyanide  solution  -     —  r  -  r  —  ^  —  —  -  • 

Au,  Ag,  Hg,  Cd 

in  sodium  sulphide  (cone.)  solution  -        —  ; 


*  Haber,  Grundriss  der  Technischen  Elektrochemie,  Leipzig,  1898. 


52  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

in  ammonium  sulphide  (dil.)  solution  au    . — 5-; 

Sb,  As,  Sn' 

Zn,  Cd,  Fe,  CO,  Ni,  Pb 
in  strong  morgamc  acids  Bi;  AS;  Sb;  Sn?  Cu?lfe  Ag?  Pd?  p^» 

Substances  are  also  added  to  the  electrolytic  solutions 
to  increase  their  conductivity. 

The  nature  of  the  added  substance  is,  in  general,  dependent 
on  the  chemical  properties  of  the  metals  in  the  solutions. 
A  fundamental  requirement,  however,  may  be  stated.  The 
substance  must  be  a  good  conductor  of  the  current  and  must 
form  no  decomposition  products  which  are  insoluble  or 
which  are  detrimental  to  the  analysis.  Alkalies,  and  acids 
which  after  their  decomposition  are  again  generated  at  the 
electrodes,  are  therefore  frequently  suitable,  as  are  also 
organic  acids,  the  decomposition  products  of  which  are  given 
off  in  a  gaseous  form.  This  last  condition  is  fulfilled  by 
oxalic  acid  especially,  which  also  on  account  of  its  ready 
solubility  is  of  special  importance  in  the  electrolysis  of  metals, 
particularly  in  the  form  of  the  double  oxalates. 

Since  solutions  of  mixed  salts  are  used  in  electrochemical 
analysis,  Faraday's  law  does  not  apply  to  the  electrolytic 
separation  of  metals  under  these  conditions;  i.e.  the  quantity 
of  metal  separated  is  not  proportional  to  the  quantity  of  elec- 
tricity passed  through  the  solution.  Especially  the  last 
traces  of  a  metal  in  a  solution  require  a  large  excess  of  current 
to  effect  their  complete  precipitation.  For  this  reason  in 
many  determinations  the  time  required  is  largely  independent 
of  the  quantity  of  metal  to  be  separated. 

The  values  of  the  decomposition  potential  of  solutions  of 
metallic  salts  have  an  important  bearing  *  on  electrochemical 

*  Kiliani,  Berg-  und  Huttenm.  Ztg.,  1883.  Freudenberg,  Zeit.  f .  phys. 
Chem.,  1893. 


ELECTROCHEMICAL   ANALYSIS.  53 

analysis,  since  they  give  the  minimum  potential-difference 
required  for  the  precipitation  of  the  metals  from  such  solu- 
tions. They  also  afford  a  means  of  quantitatively  separat- 
ing several  metals  successively  from  the  same  solution  through 
a  variation  of  the  potential-difference  of  the  primary  current. 
For  example,  zinc  is  not  precipitated  from  a  normal  zinc 
sulphate  solution  when  the  difference  of  potential  between 
the  electrodes  is  less  than  2.35  volts.  From  a  normal  silver 
nitrate  solution,  however,  the  silver  is  deposited  when  the 
potential-difference  is  equal  to  0.70  volt.  Therefore,  from  a 
mixture  of  these  two  solutions,  the  silver  can  be  separated 
by  keeping  the  potential-difference  below  2.35  volts,  the  zinc 
remaining  meanwhile  in  solution.  After  the  silver  has  been 
separated,  the  zinc  can  be  deposited  by  increasing  the  poten- 
tial-difference to  over  2.35  volts. 

Kiliani  was  the  first  to  point  out  the  importance  of  the 
difference  of  potential  in  electrolytic  separations.  Somewhat 
later,  Freudenberg,  basing  his  work  on  Le  Blanc's  studies, 
carried  out  a  careful  investigation  of  the  exact  relations. 

The  physical  state  of  the  precipitated  metal  (or  peroxide) 
is  largely  dependent  on  the  current-density  at  the  cathode 
(anode),  since  this  determines  the  number  of  the  ions  which 
in  the  unit  time  separate  as  molecules  on  a  unit  surface  of  the 
electrode,  i.e.  the  rate  of  deposit  of  the  metal  (peroxide). 
If  the  current-density  is  high  the  individual  atoms  are  de- 
posited upon  one  another  in  such  rapid  succession  that  the 
precipitated  metal  does  not  adhere  firmly  to  the  electrode, 
but  scales  off.  When  the  current-density  is  too  low  a  com- 
pact layer  does  not  form  and  the  metal  is  deposited  on  the 
electrode  in  isolated  patches,  a  condition  which  is  undesir- 
able in  quantitative  determinations.  The  current-density  is 
therefore  an  important  factor  in  quantitative  electrolysis. 

The  necessity  of  accurate  data  in  the  performance  of 


54  QUANTITATIVE    ANALYSIS   BY    ELECTROLYSIS. 

electrochemical  analysis  is  obvious,  for  unless  all  the  impor- 
tant factors  are  determined  and  recorded  the  experiment 
cannot  be  accurately  repeated. 

Since  the  determination  of  the  resistance  of  the  liquid  in 
the  cell  is  beyond  the  scope  of  analytical  work,  therefore, 
instead  of  this,  the  exact  volume  and  composition  of  the  solu- 
tion, as  well  as  the  size  and  shape  of  the  electrodes,  must  be 
stated.  In  addition  to  this,  the  difference  of  potential  be- 
tween the  electrodes,  the  current-strength  as  read  directly  on 
the  amperemeter,  and  the  calculation  of  the  current-density 
from  the  current-strength,  for  the  electrode  on  which  the 
quantitative  precipitation  has  taken  place,  must  be  given. 
All  electrical  relations  are  influenced  by  the  temperature,  so 
that  an  exact  knowledge  of  this  is  most  essential.  The  length 
of  time  required  for  the  electrolysis,  and  the  nature  of  the 
source  of  current  having  been  specified,  all  adequate  and 
necessary  data  are  at  hand  to  enable  every  one  to  repeat 
the  analysis  under  exactly  similar  conditions. 


CHAPTER  XIII. 
DETERMINATION  OF  THE  ELECTRICAL  MAGNITUDES. 

MEASUREMENT  OF  THE  CURRENT-STRENGTH. 

THE  current-strength  can  be  measured  either  by  means 
of  the  chemical  or  the  electromagnetic  action  of  the  current. 

The  chemical  instruments  are  the  gas  voltameter  and  the 
weight  voltameter. 

The  electromagnetic  instruments  are  the  galvanometer 
and  the  amperemeter. 

Gas  Voltameter  (Coulombmeter). — The  principle  upon 
which  this  instrument  depends  is  the  measurement  of  the 
volume  of  the  combined  oxygen  and  hydrogen  gas  liberated 
in  a  given  period  of  time  by  passing  the  current  to  be 
measured  through  a  solution  of  sulphuric  acid  or  sodium 
hydroxide. 

A  common  form  of  the  gas  voltameter  is  shown  in  Fig.  3. 
The  inner  vessel  A  contains  the  dilute  sulphuric  acid  (33%) 
solution  and  is  surrounded  by  an  outer  vessel  cc  containing 
water  for  cooling  purposes.  The  apparatus  is  connected 
with  the  circuit  in  which  the  current  is  to  be  measured  by 
the  binding-screws  bb  attached  to  the  platinum  electrodes  aa. 
The  hydrogen  and  oxygen  which  are  liberated  at  the  electrodes 
are  conducted  through  the  tube  ef  to  a  gas-burette,  where  the 
combined  volume  is  measured.  When  the  volume  of  the 
gas  obtained  is  reduced  to  standard  conditions,  i.e.  760  mm 
pressure  and  0°  C.,  the  strength  of  the  current  can  be  cal- 

55 


56 


QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 


culated,  since  a  current  of  1  ampere  liberates  10.44  cc  of  oxy- 
hydrogen  gas  per  minute. 

The  use  of  sulphuric  acid  as  the  electrolyte  is  objection- 
able, however,  since  owing  to  the  formation  of  ozone  and  per- 


FIG.  3. 

sulphuric  acid  at  the  anode,  and  to  other  secondary  reac- 
tions, the  results  obtained  with  it  are  often  far  from  accurate. 
A  much  more  satisfactory  electrolyte  is  a  two-per-cent. 
solution  of  sodium  hydroxide.  With  this  nickel  electrodes, 
are  employed. 

The    amperemanometer    described    by    Bredig     (Zeit.    L 
Elektrochemie,  7,  259)  is  an  extremely  simple  modification 


DETERMINATION    OF    THE    ELECTRICAL    MAGNITUDES.      57 


of  the  gas  voltameter.  It  consists  of  a  glass  vessel  a  (Fig.  4) 
partially  filled  with  a  sodium  hydroxide  (2%)  solution.  It 
is  closed  with  a  tight-fitting  rubber  stopper  through  which 
pass  nickel  wires  connecting  the  cylin- 
drical nickel  electrodes,  c  and  6,  to  bind- 
ing-screws on  the  outside,  and  also  the 
tube  de.  This  tube  is  curved  as  shown 
and  is  closed  at  e  by  a  small  rubber  stop- 
per through  which  passes  a  capillary 
tube  /.  A  small  manometer  g  contain- 
ing colored  water  is  connected  with  the 
tube  de.  The  operation  of  this  appa- 
ratus is  very  simple ;  the  escape  of  the 
gas  generated  by  the  electrolysis  is  re- 
tarded by  the  capillary  opening  through 
which  it  must  pass  and  a  pressure  is 
created  in  the  interior  which  is  shown 
by  the  manometer.  A  plug  of  loose 
cotton  wool  is  inserted  at  e  to  filter  the 
escaping  gases  and  prevent  the  capillary 
from  clogging.  The  instrument  must  be 
empirically  calibrated  by  comparison 
with  a  standard  amperemeter.  Its  range 
of  measurement  can  be  greatly  increased  by  providing  a  series 
of  capillaries  of  different  sizes.  Each  capillary  must  of 
course  be  standardised  separately.  *  The  obvious  advantage 
of  this  form  of  apparatus  is  that  when  once  calibrated  its 
readings  are  obtained  directly  in  amperes  without  calculation. 
The  chief  objection  to  the  use  of  gas  voltameters  is  that 
their  back-electromotive  force  due  to  polarisation  is  consid- 
erable (1.7-2.5  volts)  and  it  is  therefore  nearly  always  neces- 
sary to  keep  them  permanently  in  the  circuit,  which  is  often, 
very  inconvenient. 


FIG.  4. 


-58  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

Weight  Voltameter  (Coulombmeter). — In  this  instrument 
the  current-strength  is  determined  from  the  weight  of  metal 
(silver  or  copper)  deposited  in  a  given  time. 

The  silver  voltameter  is  employed  chiefly  for  determining 
the  electrochemical  equivalent  of  silver,  which  has  been 
adopted  as  the  standard  for  the  electrochemical  equivalents 
of  the  elements.  In  practice  it  usually  consists  of  a  platinum 
dish  cathode  and  an  anode  of  pure  silver.  A  silver  nitrate 
solution  is  used  as  electrolyte.*  A  description  of  its  details 
will  be  omitted. 

The  copper  voltameter,  however,  furnishes  a  very  con- 
venient and  inexpensive  apparatus  for  measuring  the  strength 
•of  currents.  In  its  simplest  form  it  consists  of  a  thin  sheet 
of  copper,  which  serves  as  cathode,  suspended  between  two 
somewhat  thicker  sheets  of  copper,  which  are  the  anodes. 
As  electrolyte  a  solution  containing  the  following  constituents 
has  proved  very  satisfactory:  f 

15  g    Copper  sulphate  (cryst.), 
5  g    Cone,  sulphuric  acid, 
5  cc  Alcohol, 
100  cc  Water.  r 

With  this   solution  the  current-density  may  vary  from  0.06 
to  1.5  ampere  per  square  decimeter  cathode  surface. J     The 


*  A  silver  voltameter  in  which  a  solution  of  silver  cyanide  and  potas- 
sium cyanide  is  used  as  electrolyte  is  described  by  Farup,  Zeit.  f.  Elek- 
trochem.,  8,  569. 

t  Oettel,  Chem.  Ztg.,  17,  543  (1893). 

J  Shepard  (Am.  Journ.  Science,  12,  49  (1901)  recommends  the  follow- 
ing solution:  a  concentrated  solution  of  copper  sulphate  (sp.  gr.  1.2) 
which  has  been  digested  with  metallic  copper  for  one  hour  at  100°  C.,  and 
to  which  a  trace  of  ammonium  chloride  has  been  added.  This  is  recom- 
mended especially  when  high  current-densities  are  employed. 


DETERMINATION    OF    THE    ELECTRICAL    MAGNITUDES.     59 

difference  of  potential  between  the  cathode  and  anode  when 
a  current  is  passing  from  is  0.1  to  0.5  volt. 

The  current-strength  is  determined  by  connecting  the 
voltameter  in  the  circuit  and  allowing  the  current  to  pass 
for  a  given  length  of  time.  Since  a  current  of  one  ampere  will 
deposit 

0.0197  gram  of  copper  per  minute 
and  1 . 181    grams  of  copper  per  hour, 

the  current-strength  can  be  calculated  by  determining   the 
increase  in  weight  of  the  cathode. 

The  copper  voltameter  is  used  chiefly  for  calibrating 
amperemeters  and  for  measuring  the  quantity  of  electricity 
passing  through  circuits  when  the  current-strength  is  sub- 
ject to  considerable  and  frequent  variations.  Since,  like  the 
other  voltameters,  it  actually  measures  coulombs  and  not 
amperes,  the  name  coulombmeter  has  been  very  appropriately 
suggested  *  for  this  type  of  instruments,  to  distinguish  them 
from  voltmeters,  which  they  in  no  way  resemble  and  with 
which  they  are  continually  confounded. 

GALVANOMETERS. 

The  term  galvanometer  is  commonly  applied  to  any  instru- 
ment used  to  detect  the  existence  of,  or  to  measure,  a  current 
by  the  deflection  produced  in  a  magnetic  needle  or  its  equiv- 
alent. 

Tangent  Galvanometer. — The  name  of  this  instrument 
is  derived  from  the  fact  that  the  current-strength  is  calcu- 
lated from  the  tangent  of  the  angle  of  deflection  of  the  needle. 
It  consists  (Fig.  5)  of  a  short  magnetic  needle  suspended  at 
the  center,  or  on  the  axis,  of  one  or  more  circular  coils  of  wire, 

*  Richards  and  Heimrod,  Zeit.  f.  physik.  Chem.,  41,  302  (1902). 


60 


QUANTITATIVE   ANALYSIS    BY    ELECTROLYSIS. 


through  which  the  current  passes.     The  plane  of  the  coil  is 
set  in  the  magnetic  meridian. 

The  current-strength  is  calculated  from  the  equation 


where  r  is  the  radius  of  the  coil,  H  is  the  horizontal  com- 
ponent of  the  earth's  magnetic  field,  n  is  the  number  of  turns 


FIG.  5. 

of  wire,  in  the  coil,  and  6  is  the  angle  of  deflection  of  the  needle 
from  the  magnetic  meridian. 

Sine  Galvanometer.  —  The   essential    difference  between 
the  sine  galvanometer  (Fig.  6)  and  the  tangent  galvanometer 


DETERMINATION    OF   THE    ELECTRICAL    MAGNITUDES.     61 

is  that  the  plane  of  the  coil  is  not  fixed  in  the  meridian  but 
<;an  be  revolved  about  a  vertical  axis.     When  a  current  is 


FIG.  6. 


passing  through  the  instrument  the  coil  is  turned  until  its 
plane  corresponds  exactly  with  that  of  the  needle.  The 
current-strength  is  then  obtained  from  the  equation 


6  being  the  angle  between  the  needle  and  the  magnetic  merid- 
ian. 

Other  Galvanometers.  —  Where  great  sensitiveness  is  re- 
quired in  a  galvanometer  it  may  be  obtained  by  combining 
two  or  more  needles  in  an  astatic  system,  i.e.  by  suspending 
them  in  the  same  plane  but  with  their  poles  reversed  and 
rigidly  connected  with  one  another,  so  that  the  effect  of 
the  earth's  magnetic  field  on  the  system  is  almost  entirely 


62 


QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 


eliminated.  This  increases  the  sensitiveness  of  the  needles, 
and  the  effect  of  the  current  on  them 
is  greatly  increased  by  surrounding 
them  with  different  coils  containing  a 
large  number  of  turns  of  the  same 
wire.  The  compound  needle,  to  which 
a  small  mirror  is  attached,  is  sus- 
pended by  a  silk  fiber,  and  its  de- 
flection is  measured  with  a  telescope 
and  scale.  A  galvanometer  of  this 
type  is  shown  in  Fig.  7. 

Another  method  of  constructing  a 
sensitive  galvanometer  is  by  sus- 
pending the  coil  through  which  the 
current  passes  in  an  intense  magnetic 
field. 

In  the  Rowland  d 'Arson val  gal- 
vanometer (Figs.  8  and  9)  the  coil  C 

is  suspended  between  the  poles,  NS,  of  a  strong  permanent 


FIG.  7. 


FIG.  8. 
magnet,  by  a  thin  strip  of  phosphor-bronze.     The  deflection; 


DETERMINATION    OF    THE    ELECTRICAL    MAGNITUDES.     63 


of  the  coil  is  measured  by  observing  the  reflection  of  the 
scale  in  the  mirror  M  through  the  telescope  attached  to  the 
instrument.  . 

Amperemeters. — The  name  amperemeter  is  applied  to  those 
galvanometers  in  which  the  current-strength  is  indicated 
directly  in  amperes,  usually  by  a 
pointer  which  moves  over  a  gradu- 
ated scale.  Owing  to  their  sim- 
plicity of  operation  they  are  by  all 
means  the  most  convenient  and 
satisfactory  instruments  for  measur- 


FIG.  9. 


FIG.  10. 


ing  the  current-strength  in  analytical  electrochemical  experi- 
ments. 

In  the  spring  amperemeter  devised  by  Kohlrausch  (Fig.  10) 
a  hollow  cylinder  of  sheet  iron  is  suspended  by  a  spiral  spring 
just  above  a  coil  of  wire  having  the  form  of  a  vertical  helix. 
When  a  current  passes  through  the  coil  the  iron  cylinder  is 
drawn  into  it  until  the  force  of  attraction  is  balanced  by 
the  tension  of  the  spring.  A  small  pointer  attached  to  the 


64  QUANTITATIVE    ANALYSIS    BY  ELECTROLYSIS. 

cylinder  moves  over  a  vertical  scale  attached  to  the  front  of 
the  instrument. 


FIG.  11. 


FIG.  12. 


In  the  amperemeter  shown  in  Fig.  11  the  coil  is  horizontal 
and  a  curved  piece  of  thin  sheet-iron  to  which  a  pointer  is 


DETERMINATION    OF    THE    ELECTRICAL   MAGNITUDES.     65 

attached  is   pivoted   eccentrically   within  it.      The   pointer 
moves  over  a  scale  on  the  front  of  the  instrument. 

Another  type  of  these  instruments,,  known  as  the  Weston 
amperemeters,  are  constructed  on  the  principle  of  the  d'Ar- 
sonval  galvanometer.  The  essential  parts  of  these  instru- 
ments (Fig.  12)  are  a  permanent  magnet,  AGB,  of  great  con- 
stancy, a  fixed  soft-iron  cylinder  C,  which  concentrates  the 
field,  and  a  coil  of  wire  d,  wound  about  an  aluminium  frame 
which  turns  freely  on  pivots  V,  V.  E  is  a  pointer  fastened 
to  the  coil  and  moving  over  an  empirically  divided  scale. 
On  breaking  the  circuit,  the  coil  is  restored  to  the  zero  posi- 
tion by  spiral  springs  S.  Only  a  very  small  portion  of  the 
current  passes  through  the  pivoted  coil,  the  greater  part 
being  deflected  through  a  shunt  of  low  resistance. 


FIG.  13. 


FIG.  14. 


Fig.  13  shows  a  standard  portable  instrument,  Fig.  14  a 
switchboard  type;  and  a  cheaper,  but  for  electrochemical 
purposes  equally  satisfactory,  instrument  is  shown  in  Fig. 
15. 

This  make  of  instrument  possesses  many  advantages;  the 


66  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

pointer  comes  to  rest  at  once  when  the  circuit  is  closed,  owing 

to  the  damping  effect  of  the  alu- 
minium frame  on  which  the  coil 
is  mounted;  the  divisions  of  the 
scale  from  zero  to  the  maximum 
are  practically  uniform  in  size; 
the  construction  permits  of  very 
accurate  reading;  and  the  re- 
FlG-  15-  sistance  is  so  low  that  these  in- 

struments can  be  introduced  into  or  removed  from  the  circuit 
without  appreciably  altering  the  strength  of  the  current. 

For  electrochemical  analysis  an  instrument  with  a  range 
of  five  amperes,  with  subdivisions  corresponding  to  one-tenth 
ampere,  will  be  found  very  satisfactory,  although  for  some 
experiments  an  instrument  of  lower  range  capable  of  meas- 
uring 0.01  ampere  may  be  required. 

MEASUREMENT  OF  THE  POTENTIAL. 

For  measuring  the  potential  a  large  number  of  instru- 
ments are  in  use,  their  suitability  depending  upon  the  accu- 
racy of  measurement  desired.  Two  instruments  are  em- 
ployed in  electrochemical  analysis,  the  voltmeter  and  the 
torsion  galvanometer;  while  for  exact  determinations  of 
differences  of  potential  and  electromotive  forces,  especially 
small  ones,  the  capillary  electrometer  and  quadrant  electro- 
meter have  been  generally  adopted. 

Voltmeter. — In  construction  the  voltmeter  (Figs.  16  and 
17)  is  essentially  a  high  resistance  galvanometer.  This  is 
accomplished  in  the  Weston  type  of  instruments  by  con- 
necting a  high  resistance  in  series  with  the  movable  coil, 
through  which  the  entire  current  furnished  to  the  voltmeter 
passes.  The  scale  of  the  instrument  is  empirically  divided 


DETERMINATION    OF    THE    ELECTRICAL    MAGNITUDES.     67 


so  that  the   position   of  the  pointer  indicates,  in  volts,  the 
difference  of  potential  between  the  binding  posts. 

In  measuring  the   difference   of  potential  between  any 
two  points  the  voltmeter  is  provided  with  a  separate  circuit  of 


FIG.  16. 


FIG.  17. 


its  own,  the  free  terminals  of  which  are  attached  to  the  points 
to  be  measured.  Since  the  resistance  of  the  instrument  is 
very  high  (several  thousand  ohms  in  the  low-range  instru- 
ments), that  portion  of  the  current  deflected  through  the 
voltmeter  circuit  may  be  neglected,  and  likewise  the  resistance 
of  the  connecting  wires. 

In  the  type  of  instruments  in  which  an  immovable  coil  acts 
on  a  movable  magnet,  the  high  resistance  is  obtained  by 
constructing  the  coil  of  a  large  number  of  turns  of  exceedingly 
fine  wire. 

Foi;  the  purposes  of  electrochemical  analysis  a  voltmeter 
having  a  scale  reading  to  10  volts,  in  fractions  of  0.1  volt,  is 
satisfactory. 

Torsion  Galvanometer. — In  this  instrument  a  light  bell- 
shaped  magnet  is  suspended  between  two  vertical  coils  by 
a  spiral  spring  attached  to  a  movable  pointer.  A  second 
pointer  is  rigidly  attached  to  the  magnet,  and  the  adjustment 


68 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


of  the  spring  is  such  that,  when  no  current  is  passing  through 
the  coils  and  the  instrument  is  properly  placed  with  respect 
to  the  magnetic  meridian,  both  pointers  indicate  zero  on  a 
horizontal  scale  calibrated  in  volts  and  located  just  below 
the  glass  cover. 

When  a  current  passes  through  the  coils  the  magnet  is 
deflected,  and  in  order  to  restore  it  to  its  original  position 
the  pointer  from  which  it  is  suspended  is  turned  in  the 


FIG.  18. 

opposite  direction.    The  position  of  this  pointer  then  indicates 
the  potential  difference  in  volts. 

These  instruments  are  made  with  high  resistance  and 
the  vibrations  of  the  magnet  are  damped  by  surrounding 


DETERMINATION    OF   THE    ELECTRICAL    MAGNITUDES.     69 

copper,  but  they  are  not  as  convenient  as  voltmetersr 
owing  to  the  fact  that  it  is  necessary  to  adjust  them  for 
each  separate  reading.  A  torsion  galvanometer  is  shown  in. 
Fig.  18. 

Capillary  Electrometer. — The  principle  upon  which  the 
action  of  this  instrument  depends  is  the  following :  If  a  small 
mercury  electrode  is  in  contact  with  dilute  sulphuric  acid,  the 
change  in  the  surface  tension  of  the  mercury  is  proportional 
to  the  change  in  the  difference  of  potential  between  the  solu- 
tion and  the  electrode,  provided  that  the  difference  of  po- 
tential is  small  (not  over  0.1  volt). 

A  very  convenient  form  of  this  electrometer  is  shown  in 
Fig.  19.  The  capillary  C  is  partially  filled  by  a  ,  short  col- 
umn of  mercury  which  is  a  portion  of 
a  larger  column  of  mercury  contained 
in  the  vertical  tube  M ,  open  at  the 
top.  The  remainder  of  the  capillary 
is  filled  with  dilute  sulphuric  acid,  as 
is  also  the  short  wide  tube  S.  The 
bottom  of  S  is  covered  with  mercury, 
which  is  connected  with  a  small  mer- 
cury contact  P  by  a  platinum  wire  fused  F 
into  the  glass  between  them.  The 

mercury  in  M  is  similarly  connected  with  an  external 
contact  N.  When  N  and  P  are  connected  with  a  small 
difference  of  potential  the  column  of  mercury  in  C  rises  or 
falls  owing  to  a  change  in  the  surface  tension  at  the  me- 
niscus. As  already  stated,  the  movements  of  the  meniscus 
are  proportional  to  the  potential-difference  when  this  does 
not  exceed  0.1  volt.  The  movements  of  the  mercury  are 
usually  observed  through  a  low-power  microscope  contain- 
ing a  transparent  glass  scale. 

In  practice  the  poles  of  a  galvanic  element  are  connected 


70  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

through  a  resistance  of  1000  ohms.  This  resistance  is  pro- 
vided with  a  series  of  contact  plugs  so  distributed  that  the 
total  resistance  of  1000  ohms  is  divided  into  nine  resist- 
ances of  100  ohms  each  and  ten  resistances  of  10  ohms  each. 
Since  it  follows  from  Ohm's  law  that  the  fall  in  potential 
through  a  conductor  is  proportional  to  the  resistance,  if  the 
total  difference  of  potential  produced  by  the  element  at  the 
ends  of  the  resistance  is  equal  to  one  volt,  the  difference  of 
potential  between  any  two  contact  plugs  having  a  resistance 
of  100  ohms  between  them  will  be  equal  to  0.1  volt,  and  the 
difference  of  potential  between  two  plugs  having  10  ohms 
between  them  will  be  0.01  volt. 

The  difference  of  potential  to  be  measured  is  now  con- 
nected by  means  of  a  movable  contact  with  alternate  plugs 
along  the  resistance,  its  polarity  being  opposed  to  that  of 
the  element  furnishing  the  standard  current.  The  move- 
ments of  the  mercury  column  of  a  capillary  electrometer 
connected  in  the  circuit  of  the  unknown  potential-difference, 
serve  to  indicate  the  point  at  which  this  difference  of  potential 
is  balanced  or  compensated  to  within  0.01  volt  by  the  known 
difference  of  potential  along  the  resistance.  The  difference 
of  potential  is  then  read  to  within  one  one-thousandth  volt 
by  noting  the  diplacement  of  the  meniscus  from  its  normal 
position  when  the  electrometer  is  short-circuited.  This 
method  is  known  as  the  Poggendorf  compensation  method. 
It  is  obvious  that  the  known  difference  of  potential  between 
the  ends,  of  the  resistance  must  always  be  greater  than  the 
difference  to  be  determined. 

Since  no  flow  of  current  takes  place  through  the  electro- 
meter under  the  conditions  of  final  measurement,  owing  to  the 
polarisation  of  the  small  mercury  electrode,  this  method  is 
particularly  suited  to  the  accurate  determination  of  the  true 
electromotive  force  of  galvanic  elements  (see  p.  46). 


DETERMINATION    OF   THE    ELECTRICAL   MAGNITUDES.     71 

Quadrant  Electrometer. — This  important  type  of  instru- 
ment (Fig.  20),  devised  by  Lord  Kelvin,  is  particularly  suited 
for  the  measurement  of  small  potential- 
differences  and  the  electromotive  forces 
of  galvanic  elements.  Its  construc- 
tion is  extremely  simple.  A  flat,  cyl- 
indrical metallic  box  is  divided  into 
four  separate  segments  (Fig.  21),  each 
of  which  is  supported  by  an  insulated 
standard.  Each  of  these  segments  is 
called  a  quadrant.*  The  opposite 
quadrants  are  connected  by  wires,  and 


FIG.  20.  FIG.  21. 

within  them  is  suspended,  by  a  quartz  fiber,  a  light  aluminium 
vane  or  needle  (N,  Fig.  21)  bearing  a  mirror.  When  the  in- 
strument is  in  use  the  needle  is  charged  to  a  relatively  high 
potential  by  a  water  battery  containing  many  elements  or 
other  suitable  appliance,  and  contact  with  the  needle  is 
effected  through  a  small  wire  attached  to  it  and  dipping 
into  a  vessel  of  concentrated  sulphuric  acid  just  beneath. 
One  pair  of  quadrants  is  grounded,  and  the  other  pair  is 
connected  with  one  pole  of  the  electromotive  force  to  be 
determined.  The  second  pole  of  the  electromotive  force  is 
connected  with  the  earth.  This  causes  a  deflection  of  the 
needle,  which  is  measured  with  a  telescope  and  scale. 


*  In  Fig.  20  one  of  the  quadrants  is  removed. 


72  QUANTITATIVE    ANALYSIS   BY   ELECTROLYSIS. 

The  instrument  may  be  used  instead  of  a  capillary  elec- 
trometer in  the  compensation  method,  or  may  be  used  inde- 
pendently after  having  been  calibrated  by  comparison  with 
a  standard  electromotive  force. 


CHAPTER  XIV. 
SOURCE  OF  CURRENT. 

THE  source  of  current  in  electrochemical  analysis  may 
be  either  chemical  or  physical.  The  former  includes  the 
galvanic  elements,  which  are  divided  into  primary  and  sec- 
ondary elements.  The  physical  sources  are  the  electromag- 
netic machines  and  the  thermopiles. 

PRIMARY  ELEMENTS. 

Leclanche*  Cell. — This  cell  consists  of  zinc,  ammonium 
chloride  solution,  and  carbon.*  Electromotive  force,  1.5 
volt.  When  it  is  closed  through  a  circuit,  zinc  goes  into 
solution,  and  hydrogen  ions,  being  more  readily  deposited 
than  the  NH4  ions,  separate  as  gaseous  hydrogen  at  the 
carbon  electrode.  Owing  to  the  absorbent  action  of  carbon 
towards  gases  a  small  quantity  of  hydrogen  can  be  deposited 
without  producing  marked  polarisation,  but  when  currents 
of  any  considerable  strength  are  furnished  by  this  element 
for  even  a  short  time  polarisation  takes  place  and  the  po- 
tential difference  rapidly  diminishes.  In  order  to  avoid  this 
disadvantage  these  elements  are  usually  modified  by  sur- 

*  In  the  descriptions  of  primary  elements  the  external  negative  pole 
will  be  given  first,  the  solution  or  solutions  next,  and  last  the  external 
positive  pole.  It  should  be  borne  in  mind,  however,  that  the  movement  of 
the  positive  electricity  in  the  interior  of  the  cells  is  from  the  negative  to 
the  positive  pole,  and  that  of  the  negative  electricity  is  in  the  opposite 
direction. 

73 


74  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

rounding  the  carbon  rod  with  a  porous  earthenware  jar  con- 
taining a  mixture  of  manganese  dioxide  and  retort  carbon, 
or  the  electrode  itself  is  composed  of  a  compressed  mixture 
of 'these  two  substances  (Fig.  22). 


FIG.  22. 

The  manganese  dioxide  exerts  an  oxidising  action  on  the 
hydrogen  ions,  and  instead  of  hydrogen  being  set  free  on  the 
electrode,  water  is  formed  in  the  solution  about  the  positive 
pole.  The  depolarising  action  of  the  manganese  dioxide  is 
limited,  however,  and  eveji  those  cells  which  contain  it  are  not 
satisfactory  as  sources  of  current  for  electroanalysis.  A  large 
number  of  cells  of  this  type  are  to  be  found  on  the  market 
under  special  trade  names.  They  differ  from  one  another 
chiefly  in  the  shapes  of  the  containing  jars,  the  electrodes, 
and  similar  minor  details.  They  all  possess  the  same  dis- 
advantages to  a  greater  or  less  extent,  and  are  suited  only  for 
ringing  electric  bells  an,d  similar  open  circuit  work. 


SOURCE    OF    CURRENT. 


75 


The  so-called  dry  cells  are  mostly  Leclanche  elements  in 
which  the  use  of  a  fluid  is  avoided  by  moistening  plaster  of 
paris,  sawdust,  or  absorbent  paper  with  the  electrolyte.  The 
negative  electrode  of  sheet-zinc  usually  forms  the  contain- 
ing vessel. 

The  internal  resistance  of  the  Leclanche  element  varies 
with  the  construction,  but  is  usually  about  0.25  ohm.  The 
internal  resistance  of  the  dry  cells  is  considerably  higher. 

Daniell  Cell. — This  cell  consists  of  zinc,  zinc  sulphate 
solution,  copper  sulphate  solution,  copper.  Its  electromo- 
tive force  is  approximately  1.1  volt. 

The  action  of  this  cell  when  furnishing  current  has  already 
been  described  on  page  44.  Its  construction  (Fig.  23)  varies 


FIG.  23. 


considerably ,  in  the  earlier  form  the  zinc  is  placed  in  a  porous 
earthenware  cup  containing  the  zinc  sulphate  solution,  and 


76  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

this  is  surrounded  by  the  copper  sulphate  solution  in  which 
is  suspended  a  cylinder  of  sheet  copper  constituting  the 
positive  pole  of  the  element.  A  later  form  (Fig.  24),  known 


FIG.  24. 

as  the  gravity  cell,  has  the  copper  electrode  at  the  bottom 
of  a  cylindrical  glass  vessel,  and  suspended  above 'it  the  zinc 
electrode.  The  copper  electrode  is  just  covered  by  a  sat- 
urated solution  of  copper  sulphate,  and  the  remainder  of  the 
vessel  is  filled  with  a  dilute  solution  of  zinc  sulphate,  the 
different  specific  gravities  of  the  two  liquids  serving  to  keep 
them  separated. 

The  internal  resistance  of  the  Daniell  cell  is  about  0.7  ohm. 

Since  the  difference  of  potential  furnished  by  this  element 
when  closed  through  a  circuit  is  very  constant,  owing  to  the 
absence  of  polarisation,  it  is  one  of  the  most  satisfactory  pri- 
mary galvanic  sources  of  current  for  electrolytical  determina- 
tions. 

Meidinger  Cell. — This  is  a  modification  of  the  Daniell  cell, 
and  consists  of  zinc,  magnesium  sulphate  solution,  copper 
sulphate  solution,  copper.  Its  electromotive  force  is  about 


SOURCE    OF    CURRENT. 


77 


1  volt.  The  reactions  in  this  cell  are  similar  to  those 
in  the  Daniell.  Zinc  passes  into  solution  to  form  zinc  sul- 
phate in  the  magnesium  sulphate  solution,  and  copper  is 
deposited  from  the  copper  sulphate  solution.  This  cell  is 
largely  used  in  the  telegraph  offices  in  Russia  and  Germany. 
Its  internal  resistance  is  about  3  ohms. 

Grove  Cell. — This  cell  (Fig.  25)  is  composed  of  zinc,  dilute 
sulphuric  acid,  nitric  acid,  platinum.  When  the  cell  is  in 
action  the  zinc  passes  into  solution  in  the  sulphuric  acid  and 
hydrogen  would  appear  at  the  platinum  electrode  were  it 
not  for  the  depolarising  influence  of  the  nitric  acid,  which 


FIG.  25. 


FIG.  26. 


oxidises  the  hydrogen  to  water.  The  platinum  electrode 
and  the  nitric  acid  are  contained  in  a  porous  cup,  which 
prevents  the  mixing  of  the  two  acids,  and  the  surface  of  the 
zinc  electrode  is  amalgamated  with  mercury  to  prevent  local 


78 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


action.  The  electromotive  force  of  this  cell  is  about  1.8  volt 
and  its  internal  resistance  approximately  0.2  ohm. 

In  the  Bunsen  cell  carbon  is  substituted  for  platinum. 
The  reactions  are  the  same,  and  the  electromotive  force  and 
internal  resistance  are  the  same  as  for  the  Grove  cell. 

In  another  type  (Fig.  26)  of  this  same  cell  the  depolar- 
ising action  is  produced  by  chromic  acid.  In  this  case  both 
the  zinc  and  the  carbon  electrode  dip  into  'the  same  solution, 
a  mixture  of  potassium  bichromate  and  dilute  sulphuric  acid. 

Cupron  Element. — This  cell  (Fig.  27)  consists  of  zinc, 
dilute  sodium  hydroxide  solution,  cupric  oxide.  The  zinc 
passing  into  solution  in  the  sodium  hydroxide  solution  tends 
to  displace  hydrogen  ions  at  the  other  pole.  The  hydrogen 
ions  react  with  the  cupric  oxide  to  form  water  and  metallic 
copper. 

The  element  has  an  electromotive  force  of  0.8  volt  and 
an  internal  resistance  of  0.05  ohm. 


FIG.  27. 


FIG.  28. 


An  element  of  this  type,  known  as  the  Edison-Lei  ande 
cell  (Fig.  28)  has  come  largely  into  use  in  the  United  States. 
These  cells  are  manufactured  with  capacities  of  from  50  to 
600  ampere-hours  and  furnish  a  very  convenient  primary 
source  of  current  for  electrochemical  experiments. 


SOURCE    OF    CURRENT.  79 


GALVANIC  SECONDARY  ELEMENTS. 

Accumulators,  or  Storage  Batteries. — Galvanic  elements 
may  be  separated  into  two  general  groups,  i.e.  reversible  cells 
and  irreversible  cells.  Reversible  cells  are  those  which  after 
having  produced  current  and  having  undergone  chemical 
change  can  be  restored  to  their  original  condition  by  passing 
through  them  a  current  in  the  opposite  direction  to  that 
which  they  have  furnished.  To  this  group  belongs  the  Daniell 
cell.  When  this  cell  is  furnishing  current  zinc  passes  into 
solution  and  copper  is  precipitated,  the  current  within  the 
cell  flowing  with  respect  to  the  positive  electricity  from  the 
zinc  to  the  copper.  If  this  cell  is  so  attached  to  a  source 
of  higher  electromotive  force  that  the  current  within  the  cell 
is  forced  to  flow  in  the  opposite  direction,  i.e.  from  the  copper 
to  the  zinc ;  then  copper  will  pass  into  solution  and  metallic 
zinc  will  be  deposited  on  the  zinc  electrode.  The  original 
condition  of  the  cell  can  thus  be  re-established. 

Irreversible  cells  are  those  in  which  the  original  conditions 
are  not  re-established  by  the  reversal  of  the  direction  of  the 
current.  Thus,  for  example,  on  passing  a  current  through  a 
partially  exhausted  Grove  cell,  in  a  direction  opposite  to 
that  of  the  current  which  it  has  furnished,  hydrogen  will 
appear  at  the  zinc  pole  and  oxygen  at  the  platinum.  In  the 
sense  in  which  this  expression  is  used,  therefore,  such  a  cell 
is  not  reversible. 

When  a  reversible  element  is  producing  a  current  of  elec- 
tricity, a  transformation  of  chemical  energy  into  electrical 
energy  is  taking  place.  When  the  direction  of  the  current 
is  reversed,  electrical  energy  is  being  transformed  into  chem- 
ical energy.  Since  the  chemical  energy  which  is  thus  stored 
up  and  accumulated  within  the  cell  may  be  again  converted 


80  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

into  electrical  energy,  this  type  of  cell  is  known  as  a  storage 
cell  or  accumulator. 

The  most  satisfactory  cell  of  this  type  is  obtained  by  the 
combination  spongy  lead,  dilute  sulphuric  acid,  lead  perox- 
ide. Its  electromotive  force  is  approximately  2  volts.  When 
this  cell  furnishes  current  the  spongy  lead  passes  into  solu- 
tion to  form  lead  sulphate,  which  is  deposited  as  an  adherent 
coating  on  the  surface  of  the  lead  electrode,  and  this  tends 
to  displace  hydrogen  at  the  other  pole,  as  a  result  of  which 
the  lead  peroxide  (Pb02)  is  reduced  to  lead  oxide  (PbO) 
which  in  turn  is  acted  upon  by  the  electrolyte  to  form  lead 
sulphate. 

In  an  external  conductor  connecting  the  two  poles,  the 
current,  with  respect  to  the  positive  electricity,  will  flow 
from  the  lead  peroxide  electrode  to  the  lead  electrode. 

The  simplest  equation  for  representing  the  reaction  *  in 
this  cell  is  the  following : 

Pb  +  Pb02  +  2H2S04  *  2PbS04  +  H20. 

If  the  current  through  the  cell  be  now  reversed,  the  lead 
sulphate  at  the  negative  (lead)  electrode  will  be  reduced  to 
spongy  lead,  and  the  lead  sulphate  at  the  positive  pole  will 
be  converted  into  lead  peroxide,  according  to  the  equation 

2PbS04+  H20  =Pb  +  Pb02  +  H2S04. 

A  large  number  of  different  types  of  this  element  have 
been,  and  still  are,  manufactured,  and  many  of  them  have 
found  very,  general  practical  application.  In  the  construc- 
tion of  accumulators  the  chief  objects  aimed  at  are  a  long 
life,  low  internal  resistance  and  a  high  "capacity''  per 

*  The  theory  of  the  lead  accumulator  has  been  the  cause  of  much  scien- 
tific argument.  Its  discussion  will  be  omitted. 


SOURCE    OF    CURRENT.  81 

pound  of  electrode  material.  By  capacity  is  meant  the  cur- 
rent furnished  by  an  accumulator  expressed  in  ampere-hours 
(product  of  amperes  and  hours). 

For  the  preparation  of  the  electrodes  two  general  methods 
are  in  use.  One,  originated  by  Plante,  depends  upon  the 
behavior  of  lead  plates  in  contact  with  sulphuric  acid  during 
electrolysis.  If  two  such  plates  are  immersed  in  dilute  sul- 
phuric acid  and  a  current  is  passed  between  them,  the  surface 
of  the  plate  connected  with  the  positive  pole  of  the  source  of 
current  is  converted  into  lead  peroxide  and  hydrogen  is 
evolved  at  the  negative  plate.  If  the  direction  of  the  current 
is  now  reversed,  the  lead  peroxide  is  reduced  to  spongy  lead 
and  the  other  plate  is  superficially  converted  into  lead  per- 
oxide. By  occasionally  reversing  the  direction  of  the  current 
and  continuing  the  operation  for  some  time,  the  action  finally 
penetrates  to  the  interior  of  the  plates,  which  are  thus  brought 
into  a  condition  suitable  for  use  in  an  accumulator.  This 
process  is  called  ' '  forming, ' '  and  is  promoted  by  the  addition 
to  the  electrolyte  of  certain  chemical  substances,  particularly 
nitrates. 

The  other  method  is  based  upon  the  application  to  a  hard 
lead  frame  or  ' '  grid, "  of  a  paste  composed  of  lead  oxide 
(litharge),  sulphuric  acid  and  sometimes  other  substances. 
The  mixture  on  standing  acquires  a  solid  consistency  and 
is  afterwards  formed  into  spongy  lead  and  lead  peroxide  by 
electrolysis  in  a  bath  of  dilute  sulphuric  acid. 

In  the  "chloride  process "  the  spongy  lead  plates  are 
prepared  by  casting  a  hard  lead  frame  around  cubes  com- 
posed of  a  mixture  of  zinc  and  lead  chlorides.  The  plates  are 
then  connected  as  cathodes  in  an  acid  bath  and  the  zinc  chlo- 
ride dissolves  out,  leaving  a  sponge-like  frame  of  lead  chloride 
which  is  converted  into  metallic  lead  by  the  electrolytic 
process. 


82  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

As  ordinarily  constructed,  an  accumulator  consists  of  a 
rectangular  jar  of  glass  or  hard  rubber,  which  contains  the 
electrolyte,  and  a  series  of  hard  lead  frames  which  carry  the 
active  materials.  Two  solid  lead  cross-bars  or  connectors 
are  rigidly  attached,  one  to  all  the  positive  plates,  the  other 
to  all  the  negative  plates,  and  the  plates  are  so  arranged 
that  the  positives  and  negatives  alternate,  one  positive  being 
placed  between  every  two  negatives,  contact  between  them 
being  prevented  by  suitable  insulators.  Electrical  connec- 
tion with  the  plates  is  effected  through  the  medium  of  long 
lead  lugs  which  are  rigidly  connected  with  the  cross-bars. 

The  capacity  of  a  given  accumulator  varies  considerably 
with  the  strength  of  the  current  which  it  is  required  to  fur- 
nish. This  is  shown  in  the  following  table,  where  the  strength 
of  the  current,  the  number  of  hours  which  it  is  supplied,  and 
the  corresponding  capacity,  is  given  for  a  certain  accumulator. 

Current-strength  Time  Capacity 

in  amperes.  in  hours.  amperes  x  hours. 

10  8  80 

14  5  70 

20  3  60 

Fig.  29  shows  a  type  of  accumulator  manufactured  by 
the  Electric  Storage  Battery  Co.  Portable  cells  (Fig.  30) 
are  also  made,  in  which  the  accumulators  are  contained  in 
sealed  rubber  jars  enclosed  in  hard- wood  cases  provided  with 
handles  and  external  binding-posts.  Various  capacities  are 
furnished. 

The  internal  resistance  of  accumulators  varies  with  the 
size  and  construction  between  0.001  and  0.02  ohm.  * 

Accumulators  furnish  the  most  satisfactory  source  of 
current  for  electrochemical  analysis.  Their  freedom  from 
obnoxious  fumes  or  vapors,  the  fact  that  their  component 
parts  require  no  renewal,  and  the  great  constancy  of  the 


SOURCE    OF    CURRENT. 


83 


potential  difference  which  they  furnish,  are  all  important  fac- 
tors which  make  them  particularly  suited  for  work  of  this 


LLLb-LLbp- 


FIG.  29. 

character.  In  the  Aachen  laboratory  four  pairs  of  accumu- 
lators have  been  constantly  in  use  since  1888,  without  need 
of  repair.  Four  accumulators  are  in  use  in  the  analytical 
laboratory,  for  which  they  have  proved  entirely  sufficient, 
and  four  in  the  author's  private  laboratory. 


84  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


REMARKS  ON  THE  USE  AND  CARE  OF  ACCUMULATORS. 

The  type  of  accumulator  which  is  ordinarily  employed 
for  the  purpose  of  quantitative  analysis  is  not  shipped  from 
the  factory  ready  mounted  for  use,  as  are  the  smaller  portable 
types  of  batteries,  but  the  glass  jars,  cathode  and  anode  plates, 


FIG.  30. 

and  the  acid  are  packed  separately.  After  the  glass  jars 
have  been  carefully  cleaned,  the  plates  are  set  in  and  the  jars 
are  filled  with  the  acid  electrolyte.  Owing  to  the  fact  that 
they  stand  transportation  much  better  under  these  condi- 
tions, the  plates  are  not  shipped  in  the  primary  state  by  the 
manufacturers,  but  are  partially  discharged  and  are  then 
dried,  the  excess  of  sulphuric  acid  having  first  been  removed 


SOURCE    OF    CURRENT.  85 

from  them.  In  order  that  they  can  be  used  as  sources  of 
current  it  is  therefore  first  necessary  to  charge  them.  This 
operation  should  be  begun  immediately  after  the  plates  have 
been  placed  in  the  electrolyte,  and  consists  in  connecting 
the  cells  with  some  suitable  source  of  current,  the  lead  per- 
oxide plates  being  connected  with  the  positive,  pole,  and  the 
spongy  lead  plates  with  the  negative  pole,  of  the  charging 
current.  The  most  favorable  conditions  for  charging  are 
always  specified  by  the  manufacturers,  and  should  always 
be  closely  complied  with  in  order  to  obtain  the  most  satis- 
factory results.  The  termination  of  the  charging  is  denoted 
by  the  evolution  of  oxygen  and  hydrogen  gases  at  the  anode 
and  cathode  respectively. 

The  purity  of  the  electrolyte  is  of  the  utmost  importance. 
In  preparing  it  only  distilled  water  and  strictly  chemically 
pure  sulphuric  acid  should  be  used.  Among  those  sub- 
stances which  are  particularly  detrimental  may  be  men- 
tioned, chlorides,  nitrates,  iron  salts,  and  arsenic.  If  only 
the  commercial  sulphuric  acid  is  available  it  can  be  partially 
purified  by  passing  hydrogen  sulphide  gas  through  it,  which 
precipitates  some  of  the  metals  as  sulphides.  The  precipi- 
tate is  allowed  to  subside,  the  clear  acid  is  decanted,  and  the 
dissolved  hydrogen  sulphide  is  expelled  by  blowing  in  air  or 
by  warming. 

When  an  accumulator  is  fully  charged  it  has  an  electro- 
motive force  of  2.2  volts,  but  when  current  is  taken  from  it 
this  falls  rapidly  to  2  volts  and  remains  practically  constant 
until  the  available  supply  of  current  is  exhausted.  The 
electromotive  force  then  falls  to  1.85  volt,  and  when  this  point 
is  reached  the  use  of  the  accumulator  as  a  source  of  current 
should  be  discontinued  and  the  process  of  recharging  it  should 
be  begun  immediately. 


86  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

To  insure  the  durability  of  accumulators,  the  following 
rules  *  must  be  observed : 

1.  They  must  be  protected  from  short-circuit. 

2.  The  maximum  rate  of  discharge  recommended  by  the 
manufacturers  must  not  be  exceeded. 

3.  An  element  must  not  be  discharged  until  its  electro- 
motive force  is  below  1.85  volt. 

4.  An  element  must  not  be  allowed  to  stand  in  an  un- 
charged condition;   and  when  not  in  use  it  must  be  charged 
at  least  once  in  every  three  or  four  months. 

5.  Violent  shaking  must  be  avoided,  since  this  causes  the 
active  material  to  become  loosened. 

During  the  process  of  charging,  and  especially  during  super- 
charging, when  gas  is  being  generated  at  the  cathode  and 
anode,  an  irritating  vapor,  which  produces  a  choking  sensa- 
tion in  the  throat  and  exerts  a  very  corrosive  action  on 
exposed  metal  surfaces,  is  given  off  by  the  accumulators. 
This  can  be  entirely  prevented  by  pouring  a  thin  layer  of 
pure  mineral  oil  on  the  surface  of  the  electrolyte.  An  oil 
of  the  consistency  of  the  so-called  "cylinder  oil"  will  be 
found  most  satisfactory.  This  treatment  possesses  other 
important  advantages;  e.g.  the  creeping  of  the  electrolyte 
along  the  lugs  to  the  connectors  is  entirely  prevented;  the 
electrolyte  does  not  evaporate;  single  cells  can  be  moved 
and  carried  about  with  much  greater  freedom,  since  the 
viscosity  of  the  oil  greatly  retards  slopping.  In  batteries 
containing  a  large  number  .of  single  cells  connected  in  series, 
leakage  of  current  from  one  cell  to  another  is  reduced  to  a 
minimum  not  otherwise  obtainable. 

For  charging  accumulators  the  current  may  be  furnished 
by  a  dynamo,  a  thermopile,  or  by  primary  elements. 

*  Anleitung  zu  elektrochemischen  Versuchen  von  Dr.  Felix  Ottel^ 
1894. 


SOURCE    OF    CURRENT.  87 

If  the  charging  current  is  taken  from  a  dynamo,  the  number 
and  arrangement  of  the  separate  accumulators  should  be 
such  that  the  electromotive  force  of  the  dynamo  current  will 
exceed  that  of  the  accumulators  by  a  small  amount.  A 
rheostat,  for  further  regulating  the  current,  should  be  in- 
cluded in  the  circuit,  which  should  also  contain  an  ampere- 
meter for  showing  the  strength  of  the  current  used  in  charging. 
The  maximum  charging  current  recommended  by  Hihe 
manufacturers  for  the  particular  model  of  accumulator  in  use 
should  never  be  exceeded. 

If  the  number  of  accumulators  is  not  sufficient  to  nearly 
compensate  the  primary  potential  of  the  charging  current,  a 
rheostat  with  sufficient  resistance  to  reduce  the  charging 
current  to  the  desired  strength  may  be  used. 

The  direct  current  furnished  in  many  places  for  electric 
lighting  can  be  very  conveniently  used  for  charging  accumu- 
lators. Where  only  three  or  four  cells  are  to  be  charged, 
and  this  number  is  sufficient  for  all  ordinary  electroanalytical 
purposes,  they  may  be  connected  together  in  series  (p.  96) 
and  charged  from  the  lighting  circuit  by  interposing  a  suitable 
incandescent  lamp  resistance.  Thus,  for  example,  if  the  nor- 
mal charging  rate  of  the  accumulators  is  5  amperes,  and  the 
difference  of  potential  of  the  lighting  circuit  is  110  volts,  the 
charging  may  be  effected  by  inserting,  in  the  circuit  contain- 
ing the  accumulators,  a  resistance  consisting  of  ten  sixteen- 
candle-power  lamps  connected  in  parallel.  Since  each  16  CP. 
lamp  requires  a  current  of  one-half  ampere,  the  total  current 
passing  the  accumulators  will  be  equal  to  5  amperes. 

Since  it  is  perfectly  feasible  to  charge  accumulators  with 
a  current  below  the  normal,  the  simplest  and  most  convenient 
method  is  often  to  connect  the  accumulators  in  parallel  in  a 
loop  of  the  main  conductor  supplying  a  number  of  lights. 
When  the  lights  are  turned  on  the  current  will  pass  through 


88  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  cells,  and  the  effect  of  the  back-electromotive  force  of 
the  cells  will  not  appreciably  diminish  the  brilliancy  of  the 
lights  in  the  circuit. 

If  a  thermopile  or  primary  galvanic  elements  are  used  for 
charging  accumulators,  it  is  usually  necessary  to  connect 
the  accumulators  in  parallel  (p.  96).  This  method  of  charg- 
ing is  extremely  tedious  and  inconvenient,  and  should  be 
resorted  to  only  under  the  force  of  necessity. 

When  no  dynamo  or  circuit  suitable  for  charging  is  to 
be  had  on  the  premises,  it  is  best  to  use  the  portable  type 
of  accumulator  shown  in  Fig.  30,  which  can  be  charged  out- 
side the  building  at  some  point  where  current  is  available. 

It  is  very  desirable  to  have  a  pair  of  lead  safety-fuses 
inserted  in  the  circuit  with  the  accumulators  to  protect  against 
excessively  high  currents.  The  capacity  of  these  fuses 
should  be  such  that  they  will  l  i  blow-out ' '  when  the  current- 
strength  exceeds  the  maximum  recommended  for  the  given 
accumulators. 

PHYSICAL  METHODS  OF  PRODUCING  THE  CURRENT. 

Dynamos. — The  production  of  current  by  dynamo  ma- 
chines depends  upon  the  principle  that  when  a  conductor 
is  moved  through  a  magnetic  field  a  current  is  set  up  in  the 
conductor,  the  direction  of  the  current  being  such  as  to  oppose 
the  change  which  produced  it.  The  induced  electromotive 
force  is  proportional  to  the  number  of  lines  of  force  which  are 
cut  per  second  by  the  conductor. 

In  dynamos,  as  they  are  commonly  constructed,  the  mag- 
netic field  is  produced  between  the  poles  of  powerful  electro- 
magnets, and  the  conductors  in  which  the  current  is  generated 
consist  of  insulated  copper  wires  wound  about  an  iron  core 
which  serves  to  concentrate  the  magnetic  field.  The  core  is 
attached  to  a  shaft  or  spindle  supported  by  lubricated  bear- 


SOURCE    OF    CURRENT. 


89 


ings,  and  through  the  application  of  power  to  the  shaft  the 
core  and  attached  conductors  are  rapidly  rotated  in  the 
magnetic  field.  In  order  to  secure  a  direct  current  of  prac- 
tically uniform  strength  and  potential,  the  moving  conductor 
is  divided  into  a  number  of  sections  which  are  separately 
connected  to  insulated  bars  of  copper  placed  in  the  form  of  a 
ring  about  the  spindle.  Metallic  brushes  attached  to  the 
frame  of  the  machine  make  electric  contact  with  the  copper 
bars  on  the  spindle. 

The  electromagnets  which  produce  the  field  are  known 
as  the  field-magnets;  the  iron  core  with  the  attached  con- 
ductors is  called  the  armature;  and  the  ring  built  up  of  copper 
bars  is  called  the  commutator. 

Direct-current  dynamos  are  classified,  according  to  the 
manner  in  which  the  field  magnets  are  excited,  as : 

1.  Series  dynamos,  in  which  the  whole  current  from  the 
armature  passes  through  the  coils  of  the  field-magnets,  their 
resistance  having  been  made  as  small  as  possible  (Fig.  31). 

2.  Shunt  dynamos,  in  which  only  a  portion  of  the  current 


L  L         I 


FIG.  31. 


FIG.  32. 


FIG.  33. 


is  allowed  to  pass  through  the  coils  of  the  field-magnets  by 
joining  them  in  parallel  with  the  main  circuit  (Fig.  32). 
3.  Compound  dynamos,  in  which  the  field-magnets   are 


90  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

excited  partly  by  a  few  turns  of  wire  in  series  with  the  arma- 
ture, and  partly  by  a  coil  connected  in  parallel  with  the  main 
circuit  (Fig.  33). 

Series  dynamos  are  used  where  the  conditions  of  the  ex- 
ternal circuit  are  unchanging.  Shunt  and  compound  dyna- 
mos are  used  where  changes  in  the  external  circuit  are  de- 
sirable or  unavoidable. 

Shunt  dynamos  (Fig.  34)  are  the  most  satisfactory  type 


FIG.  34. 

for  electrochemical  purposes,  since,  by  introducing  a  vari- 
able resistance  in  the  circuit  of  the  field  coils  (Fig.  32) ,  the 
strength  of  the  field,  and  correspondingly  the  electromotive 
force  of  the  external  circuit,  can  be  changed  and  regulated. 

When,  after  having  been  at  rest,  the  dynamos  are  started, 
the  weak  residual  magnetism  remaining  in  the  field-magnets 
serves  to  build  up  the  current  in  the  armature. 

The  power  for  running  a  dynamo  may  be  taken  from  an 
engine  or  may  be  furnished  by  an  electric  motor.  If  the 
former  is  used,  the  most  efficient  results  are  obtained  by  con- 
necting the  engine  and  dynamo  directly  together,  and  trans- 
mitting the  power  by  a  single  shaft  from  one  to  the  other. 
The  small  dynamos,  such  as  are  suitable  for  currents  used  in 


SOURCE    OF    CURRENT. 


91 


electroanalysis,  may  be  driven  by  belts  attached  to  pulleys 
on  suitable  counter-shafts.  Where  an  electric  motor  is  em- 
ployed, the  dynamo  can  be  connected  with  it  by  a  belt,  but 
in  such  cases  it  is  usually  more  satisfactory  to  use  a  motor- 


FIG.  35. 

dynamo  (rotary  transformer)  which  may  consist  of  a  motor 
and  dynamo  mounted  on  the  same  frame  and  connected  by 
a  single  shaft  (Fig.  35),  or  of  a  single  set  of  field-magnets  and 
an  armature  containing  two  independent  windings  (Fig.  36). 

The  most  convenient  plan  in  electrochemical  analysis  is 
to  use  the  current  from  a  dynamo  only  for  charging  storage 
batteries,  but  it  is  perfectly  practical  to  use  it  directly  as  will 
be  described  later  in  Chapter  XV.  If  it  is  to  be  used  directly, 
an  electromotive  force  of  about  10  volts  will  meet  every  re- 
quirement. If  it  is  to  be  used  for  charging  accumulators, 
its  capacity  will  depend  upon  the  number  and  size  of  these. 

The  horse-power  required  for  driving  a  given  dynamo  can 
be  approximately  determined  by  multiplying  the  current  in 


92 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


amperes  by  the  potential  in  volts,  and  dividing  the  product 
by  625. 

Thermopiles. — If  two  strips  of  dissimilar  metals  are  con- 
nected at  their  ends,  and  one  of  the  junctions  thus  formed  is 


FIG,  36. 

maintained  at  a  different  temperature  from  the  other,  an 
electric  current  will  be  produced  in  this  circuit.  If  the  strips 
consist  of  copper  and  bismuth,  a  continuous  current  will  flow 
from  the  bismuth  to  the  copper  across  the  warmer  junction, 
the  work  done  by  the  current  being  mechanically  equivalent  to 
the  heat  which  is  absorbed  at  the  warmer  junction.  The 
electromotive  force  of  such  a  couple  is  small,  but  by  combin- 
ing a  sufficient  number  of  such  pairs  in  series,  it  can  be  in- 
creased to  a  point  where  the  combination  is  suitable  as  a 


SOURCE    OF    CURRENT. 


93 


source  of  primary  current.  An  arrangement  of  this  sort  is 
called  a  thermopile,  and  the  separate  couples  are  called 
thermo-elements . 

Practical  thermopiles  have  been  designed  by  Clamond, 
Noe7  and  Giilcher. 

The  general  appearance  of  Giilcher 's  thermopile  is  shown 
in  Fig.  37.  The  two  metals  forming  the  elements  are  a  nickel 


FIG.  37. 

alloy  (known  as  argentan)  and  a  special  alloy  the  chief  ingre- 
dient of  which  is  antimony.  The  heating  is  done  by  gas, 
and  the  separate  elements  are  so  constructed  that  a  mixture 
of  gas  and  air  is  conducted  through  an  argentan  tube  and 
burns  with  a  non-luminous  flame  just  at  the  point  where  the 
top  of  the  tube  is  in  intimate  metallic  contact  with  a  flat  bar 
of  the  antimony  alloy.  These  points  of  contact  between  the 
tubes  and  the  antimony  bars  constitute  the  hot  junctions, 
and  the  cool  junctions  lie  at  those  points  where  each  bar  of 
antimony  is  connected  with  the  bottom  of  the  argentan  tube 
of  the  next  adjoining  element.  Contact  between  the  antimony 
bars  and  the  bottoms  of  the  burner-tubes  is  effected  by  means 
of  thin  plates  of  sheet  copper  which  offer  a  large  surface  for 
the  radiation  of  heat  and  thus  tend  to  maintain  the  cooler 
junctions  at  a  relatively  low  temperature.  The  separate 


94  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

elements  are  assembled  in  series  in  two  rows  on  an  insulated 
iron  frame,  and  the  mixture  of  gas  and  air  is  supplied  to  them 
from  a  large  pipe  running  directly  beneath  them. 

The  electromotive  force  of  the  thermopile  varies  with 
changes  in  the  temperature  and  since  this  is  to  a  considerable 
extent  dependent  on  the  gas  pressure,  the  latter  must  be  care- 
fully regulated  if  a  constant  electromotive  force  is  required. 
Of  the  various  regulators  constructed  for  this  purpose,  that 
designed  by  Danneel  *  is  the  simplest  and  most  efficient. 

The  construction  of  this  instrument  is  shown  in  the  ad- 
joining sketch  (Fig.  38).  The  solenoid  S  is  connected  by 
wires  to  the  binding-posts  of  the  thermopile,  and 
the  gas  supplied  to  the  thermopile  passes  through 
the  regulator,  entering  at  B  and  leaving  at  A. 
The  solenoid  fits  over  a  glass  tube,  within  which  is 
a  permanent  magnet  M  suspended  by  a  spiral 
spring  capable  of  adjustment.  An  increase  in 
the  electromotive  force  of  the  thermopile,  due 
to  a  rise  in  temperature  at  the  junctions,  causes 
more  current  to  flow  through  the  coil,  and  the 
magnet  is  drawn  further  into  it.  The  lowering 
of  the  magnet  forces  a  disc  down  on  the  orifice  of 
the  gas  supply  pipe  and  reduces  the  supply  of  gas 
FIG.  38.  passing  to  the  thermopile.  C  is  a  thumb-screw 
by  which  a  second  opening  from  B  to  A  can  be  so  regulated 
that  just  'sufficient  gas  passes  through  it  to  keep  the  burners 
of  the  thermopile  from  being  extinguished  when  the  other 
valve  is  completely  closed. 

*Zeit.  f.  Elektrochemie,  3,  81  (1896-97). 


CHAPTER   XV. 

METHODS  OF  REGULATING  THE  CURRENT-STRENGTH 
AND  POTENTIAL. 

THE  relations  between  the  current-strength,  the  difference 
of  potential,  and  the  resistance  are  expressed  by  the  equation 

Difference  of  potential 
Current-strength  =  —       — ~ — ^—  —  • 

Resistance 

In  any  given  system  consisting  of  a  source  of  current  and 
an  external  circuit,  the  above  equation  in  order  to  express 
the  exact  relations  may  be  written 


R+r  ' 

in  which  e  represents  the  electromotive  force  of  the  source 
of  current,  R  the  resistance  of  the  external  circuit,  and 
r  the  internal  resistance  of  the  source  of  current.  In  the 
case  of  galvanic  cells,  the  factor  r  represents  the  internal 
resistance  of  the  cell,  and  where  the  source  of  current  is  a 
dynamo,  r  represents  the  resistance  of  the  circuit  through 
which  the  current  passes  within  the  dynamo. 

In  any  given  circuit  having  a  definite  external  resistance, 
the  simplest  method  of  altering  the  current-strength  is  by  a 
direct  change  in  the  difference  of  potential.  Where  the 
source  of  current  is  a  galvanic  element,  the  difference  of  po- 
tential can  be  increased  by  connecting  two  or  more  of  these 
elements  in  series. 

95 


96 


QUANTITATIVE  ANALYSIS    BY    ELECTROLYSIS. 


The  process  of  connecting  two  galvanic  elements  in  series 
consists  in  connecting  the  positive  pole  of  the  one  to  the  nega- 
tive pole  of  the  other.  The  current  is  then  taken  from 
the  two  remaining  poles.  Any  number  of  elements  can  be 
connected  in  this  manner,  as  shown  in  Fig.  39,  and  the 


FIG.  39. 

electromotive  force  of  the  combination,  when  the  elements  are 
all  alike,  is  equal  to  the  electromotive  force  of  one  element 
multiplied  by  the  total  number  of  elements  thus  connected. 
When  two  or  more  elements  are  connected  in  series,  the 
current-strength  in  the  circuit  is  represented  by  the  equation 


c xe 

R  +  xr' 


in  which  x  is  the  number  of  elements  and  e  is  the  electromo- 
tive force  of  each  element. 

Another  method  of  increasing  the  current-strength  in  a 
circuit  is  by  connecting  two  or  more  elements  in  parallel. 


FIG.  40. 


This  consists  in  connecting  all  of  the  positive  poles  of  the 
several  elements  to  one  conductor,  and  all  of  the  negative 
poles  to  another,  as  shown  in  Fig.  40.  The  external  circuit 
is  then  connected  with  the  two  conductors.  The  electronic- 


REGULATING  THE  CURRENT-STRENGTH  AND  POTENTIAL.  97 

live  force  of  such  an  arrangement  is  the  same  as  the  electro- 
motive force  of  a  single  cell,  but  the  internal  resistance  is 
equal  to  the  resistance  of  one  element  divided  by  the  total 
number  of  elements.  The  current-strength  under  these  con- 
ditions is  determined  by  the  equation 


The  effect  of  this  arrangement  on  the  current-strength  is 
largely  dependent  on  the  relative  values  of  the  internal  and 
external  resistances,  since  the  smaller  the  value  of  R  with 
respect  to  r,  the  greater  will  be  the  increase  of  the  current 
produced  by  thus  connecting  the  elements  in  parallel. 

Another  means  for  altering  the  current-strength  in  a 
circuit  is  afforded  by  the  introduction  or  removal  of  resist- 
ance. This  is  usually  effected  through  the  use  of  rheostats 
or  resistance-boxes. 

The  name  rheostat  is  given  to  any  appliance  containing  a 
liquid  or  metal  conductor  the  electrical  resistance  of  which 
can  be  altered  at  pleasure.  Instruments  of  this  sort,  con- 
structed on  various  patterns,  can  be  purchased  from  dealers 
in  electrical  instruments.  Only  a  few  of  these  will  be  here 
mentioned. 

Fig.  41  shows  a  general  form  of  instrument  known  as  a 
' '  resistance-box. ' '  The  resistances,  in  the  form  of  coils  of 
fine  wire,  are  contained  in  the  interior,  and  the  terminals  of 
these  coils  are  attached  to  metal  blocks  fastened  to  the  top 
of  the  box.  By  inserting  metal  plugs  between  adjoining 
blocks  the  resistances  can  be  short-circuited  and  removed 
from  the  path  of  the  current.  As  ordinarily  constructed 
these  instruments  are  ill  adapted  to  use  in  a  chemical  labora- 
tory, .because  the  exposed  metal  parts  are  quickly  attacked 


"98  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

.by  acid  vapors.     This  difficulty  has  been  met  by  the  use  of 


FIG.  41. 

mercury  contacts,  instead  of  plugs,  to  connect  the  metal 
plates.     An  instrument  of  this  sort;  made  at  the  author's 


FIG.  42. 


suggestion  by  Fraas  Brothers  of  Wunsiedel  (Germany),  is 
:shown  in  Fig.  42.     This  is  so  constructed  as  to  give  resist- 


REGULATING   THE    CURRENT-STRENGTH    AND    POTENTIAL.    99 


ances  from  0.5  to  80  ohms,  in  multiples  of  0.5  ohm.  Con- 
tinued use  has  established  the  fact  that  such  a  rheostat  is 
extremely  convenient  and  satisfactory. 

Another  commercial  form  of  rheostat  is  shown  in  Fig.  43. 
The  variable  resistance  is  a  cir- 
cular coil  of  wire  one  end  of 
which  is  attached  to  one  of  the 
binding  posts  shown  at  the 
corners  of  the  base.  The  other 
post  is  connected  with  the 
pivot  of  a  movable  lever.  To 
the  end  of  the  lever  is  fastened 
a  sliding  contact  which  presses 
on  the  coil  of  wire.  In  the 


FIG.  43.  FIG.  44. 

rheostat  shown  in  Fig.  44,  the  lever  can  be  moved  over  a 
number  of  metal  studs  arranged  in  a  semicircle  on  an 
insulated  base.  One  terminal  of  the  circuit  is  connected 
with  the  lever,  the  other  with  a  coil  of  wire  attached  to  the 
frame.  By  moving  the  lever  over  the  studs  the  current  is 
forced  to  pass  through  any  number  of  the  separate  sections 
into  which  the  resistance  coil  is  divided. 

A  commercial  type  of  rheostat  much  used  in  the  United 
States  is  known  under  the  trade-name  of  "  Iron-Clad. "  In 
these  rheostats  the  wire  resistance  is  fused  into  a  protecting 
enamel,  and  the  possibility  of  corrosion  or  mechanical  injury 
is  entirely  excluded. 


100 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


It  is.  often  possible  to  construct  simple  rheostats,  which 
serve  all  the  purposes  required  in  the  ordinary  course  of 
electrochemical  analysis,  rather  than  to  purchase  the  more 
costly  ones  from  dealers. 

An  arrangement  *  of  this  sort  is  shown  in  Fig.  45.     It 


FIG.  45. 

consists  of  a  light  wooden  frame,  in  the  form  of  a  parallelo- 
gram, back  and  forth  across  which  is  strung  a  galvanised 
iron  wire,  supported  on  small  porcelain  or  glass  insulators. 
If  currents  of  low  potential  only  are  to  be  used,  the  in- 
sulators can  be  entirely  dispensed  with,  and  the  wire  can 
be  passed  simply  through  iron  screw-eyes  inserted  in  the 
woodwork.  One  end  of  the  wire  is  attached  to  a  binding- 
post  with  which  one  terminal  of  the  circuit  can  be  connected, 
and  the  other  terminal  of  the  circuit  is  fastened  to  a  brass 
clamp  (a)  which  can  be  clamped  on  the  iron  wire  at  any 
point  along  it.  The  resistance  which  can  thus  be  introduced 
is  only  limited  by  the  total  resistance  of  the  iron  wire,  and  any 
fraction  of  this  total  can  be  inserted  by  attaching  the  clamp 
at  the  proper  position.  The  frame  can  be  attached  to  the 
wall  and  the  strands  of  wire  can  run  up  and  down  across  it, 
in  which  case  contact  can  be  made  with  the  bottoms  of  the 
separate  strands.  In  many  cases  such  an  arrangement  is 
extremely  convenient. 

*  Edgar  F.  Smith,  Electro-Chemical  Analysis,  1894. 


HEGULATING    THE    CURRENT-STRENGTH    AND    POTENTIAL.  101 

Another   simple    and   very    convenient    arrangement   is 
shown  in  Fig.  46.     Two  separate  resistances  composed  of 


FIG.  46. 

narrow  strips  of  sheet-zinc  (or  galvanised  sheet-iron)  are 
fastened  to  the  opposite  sides  of  a  wooden  frame,  and  one  end 
of  each  of  these  strips  is  connected  with  a  separate  binding- 
post.  To  these  binding^posts  the  terminals  of  the  circuit  are 
connected,  and  the  resistance  between  them  is  varied  by 
moving  a  sliding  clamp  (S)  which  is  in  contact  with  both 
strips,  along  the  top  of  the  frame.  Fig.  46  (A)  shows  the 
end  view  of  such  a  rheostat,  and  B  in  the  same  figure  shows 
how  the  strips  can  be  cut  from  a  sheet  of  zinc.  The  thickness 
of  the  sheet-zinc  used  and  the  width  of  the  strips  determine 
the  resistance  for  a  given  length  of  conductor.  The  frame 
is  provided  with  feet  so  that  it  can  stand  upright,  and  it  can 
be  placed  on  the  floor  in  the  neighborhood  of  the  work-bench. 
The  plan  of  a  simple  rheostat  described  by  Oettel  *  is 
given  in  Fig.  47.  Spirals  of  wire  forming  the  variable 
resistance  are  attached  to  a  wooden  frame  at  the  top  and  to 
heavier  copper  wires  at  the  bottom.  The  ends  of  the  copper 
wires  dip  into  cavities  in  the  base-board  containing  mercury. 
The  ends  of  the  wire  resistance  are  connected  with  binding- 


*  Anleitung  zu  Elektrochemischen  Versuchen,  1894. 


102 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


posts,  by  which  the  rheostat  is  inserted  in  the  circuit.  The 
most  convenient  arrangement  is  obtained  by  having  the 
length  of  the  wire  in  the  different  spirals  in  the  ratios 


FIG.  47. 

1:2:4:8:16:32',  etc.,  by  which  any  number  of  units  of  re- 
sistance can  be  introduced.  The  resistance  of  the  circuit 
passing  through  the  rheostat  is  reduced  at  will  by  placing 
copper- wire  bridges  (a)  between  adjacent  mercury  cups. 
If  a  fine  adjustment  of  the  resistance  is  desired,  this  can 
be  accomplished  by  stretching  a  wire,  of  a  length  slightly 
greater  than  that  in  the  smallest  spiral,  in  the  form  of  a  loop 
with  parallel  sides  along  an  upright  wooden  strip  attached  to 
the  end  of  the  frame  supporting  the  spirals.  This  loop  carries 
a  slider  (S)  in  contact  with  both  strands,  and,  by  moving  this 
slider  up  or  down,  a  very  delicate  adjustment  of  the  resistance 
can  be  effected. 

The  methods  for  regulating  the  potential  of  the  current, 
which  are  more  or  less  independent  of  the  electromotive 
force  of  the  source  of  supply,  depend  in  general  upon  the 
fact  that  the  fall  in  potential  along  any  conductor  through 
which  a  current  is  passing  is  proportional  to  the  resistance. 


KEGULATIXG    THE    CURRENT-STRENGTH    AND    POTENTIAL.   103 

Thus,  for  example,  if  A  (Fig.  48)  is  the  source  of  a  current 
flowing  through  the  circuit  ABC  A,  and  the  conditions  are 
such  that  the  difference  of  potential  between  B  and  C  is- 


FIG.  48. 

equal  to  x  volts,  the  difference  of  potential  between  any  two 
points  on  the  conductor  BC  will  be  represented  by  the 
equation 

V^y'jM. 

Yl      Vz~  R  ' 

where  R'  is  the  resistance  between  the  two  points  in  question 
and  R  is  the  total  resistance  of  the  conductor  BC.  If  x  is- 
equal  to  10  volts,  and  the  conductor  BC  is  100  centimeters 
long  and  of  uniform  material  and  cross-section,  the  differ- 
ence of  potential  between  any  two  points  1  cm  apart  will 
be  0.1  volt. 

By  connecting  one  electrode  of  an  electrolytic  cell  with 
the  point  B  and  the  other  electrode  with  some  point  along 
the  conductor  BC,  any  difference  of  potential  less  than  10 
volts  can  be  maintained  between  the  two  electrodes. 

An  arrangement  of  this  sort,  where  a  second  circuit  is 
connected  in  parallel  with  another  circuit,  is  called  a  shunt 
connection,  and  the  second  circuit  is  known  as  the  shunt 
circuit. 

A  shunt  circuit  attached  to  the  conductor  BC  is  shown 
in  Fig.  49.  If  R  represents  the  total  resistance  of  the  circuit, 
Rl  the  resistance  of  the  portion  CAB  (consisting  of  the  re- 
sistance of  the  conductors  BA  and  CA  and  the  internal 
resistance  of  the  source  A),  R2  the  resistance  of  BD,  R3  the 


104  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

resistance  of  DC,  R^  the  resistance  of  the  shunt  circuit  con- 
taining the  electrolytic  cell  E,  and  if   e  is  the  electromotive 

-Kl v 


FIG.  49. 


force  of  the  source  of  current,  then  the  current  C  flowing 
through  the  entire  circuit  will  be  represented  by  the  equation 


and  the  value  of  R  is  given  by  the  equation 


R2       ? 

The  values   for  the  current-strengths  through  the    parallel 
circuits  BD  (c2)  and  BED  (c4)  are  determined  by  the  equations 

riT>  rrp 

\jL\jt  vy  xLo 


Finally,  the  difference  of  potential  (x)  between  the  points 
B  and  D  can  be  calculated  from  the  equation 

.    #2#4 

• 


Prom  an  examination  of  these  equations  it  will  be  evident 
that  when  R2  +  R3  is  large  with  respect  to  Rlt  and  R4  is  large 
with  respect  to  R2,  the  current-strength  through  the  main 


HEGULATING    THE    CURRENT-STRENGTH    AND    POTENTIAL.   105 

circuit  will  be  but  little  altered  by  connecting  or  disconnecting 
the  shunt  circuit  BED. 

The  simplest  practical  arrangement  for  thus  utilising  the 
variable  difference  of  potential  which  can  be  obtained  in  a 
shunt  circuit  is  suggested  by  Fig.  49.  The  conductors  BA 
and  AC  can  consist  of  stout  copper  wires,  and  the  points  B 
and  C  can  be  connected  by  a  wire  of  German-silver  having 
a  resistance  of  say  10  ohms.  If  the  source  of  current  is  two 
accumulators  connected  in  series,  a  difference  of  potential 
of  approximately  4  volts  will  exist  between  B  and  C.  By 
means  of  a  sliding  contact  on  EC  any  difference  of  potential 
less  than  4  volts  can  be  brought  into  action  in  the  cell  E. 

A  simple  instrument  has  been  designed  by  the  author 
for  this  purpose  (Fig.  50). 

The  current  from  the  battery  enters  at  a,  passes  through 
the  German-silver  resistance  NN,  and  returns  to  the  battery 


FIG.  50. 


through  b.  In  making  electrolytic  determinations  the  plat- 
inum dish  serving  as  cathodes  are  connected  with  any 
one  of  the  binding-posts  numbered  from  1  to  20,  while  the 


106 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


platinum  anodes  are  connected  with  the  binding-posts  marked 
with  the  +  sign.  With  this  apparatus  eight  different  opera- 
tions can  be  conducted  simultaneously. 


Another   larger   ana   more   complicated   instrument   for 
the  same   purpose  was  constructed  for  the  author's  labora- 


REGULATING    THE    CURRENT-STRENGTH    AND    POTENTIAL.  107 

tory  by  the  firm  of  Siemens  &  Halske  (Fig.  51).  With 
this  instrument  a  large  number  of  electrolytic  determina- 
tions, requiring  currents  differing  in  strength  and  potential, 
can  be  carried  on  independently  of  one  another.  The  current 
from  a  dynamo  with  a  potential  of  10  volts  enters  by  the 
cables  attached  to  the  ends  of  the  resistance  MM^  This 
resistance  is  composed  of  brass  wire-gauze  in  strips  arranged 
in  a  zigzag  across  the  top  of  the  base  of  the  instrument 
and  is  divided  into  20  divisions  by  the  contact -bars 
0,  1,  2,  3  •  •  •  20,  to  which  it  is  fastened  at  equal  intervals. 
Since,  as  already  stated,  the  total  difference  of  potential  is 
equal  to  10  volts,  the  difference  of  potential  between  any  two 
adjacent  contact-bars  will  be  equal  to  one-twentieth  of  the 
total,  or  ^  volt.  Connection  with  the  separate  contact-bars 
is  effected  by  wires  attached  to  the  binding-posts  Klf  K2,  etc., 
which  slide  on  the  galvanised  iron  strips  Slt  S2,  etc.,  and  the 
latter  are  connected  with  the  blocks  Wl}  W2,  etc.  The  positive 
terminal  of  the  dynamo  is  connected  with  the  long  contact- 
bar  M2  at  the  lower  edge  of  the  base-board.  The  cells  in 
which  the  electrolytic  determinations  are  conducted  are  con- 
nected between  the  bar  M 2  and  the  blocks  Wly  W2,  etc.  The 
total  current  passed  through  the  resistance  was  60  amperes. 
A  voltmeter  (G)  attached  to  the  ends  of  the  resistance  indi- 
cates the  total  difference  of  potential. 

Electrochemical  analysis  can  often  be  very  conveniently 
and  satisfactorily  carried  out  by  the  use  of  the  current  sup- 
plied for  lighting  purposes.  The  direct  current  used  in  the 
United  States  for  incandescent  lighting  is  usually  distributed 
by  what  is  known  as  the  three-wire  system.  In  this  system 
the  circuit  consists  of  three  wires  which  for  the  purpose  of 
explanation  will  be  designated  as  a,  6,  and  c.  Under  normal 
conditions  the  difference  of  potential  maintained  between 
the  wire  a  and  the  wire  b  is  110  volts,  a  being  positive  with 


108 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


respect  to  b. 


The  difference  of  potential  between  b  and  c  is 
also  110  volts,  but  c  is  negative  with 
respect  to  b.  The  electrical  relations 
between  the  wires  may  be  considered 
as  of  the  same  nature  as  that  between 
the  three  wires  a',  b',  and  c'  (Fig.  52), 
where  a'  is  connected  with  the  posi- 
tive pole  of  a  galvanic  element,  b'  to 
the  negative  pole  of  the  same  element, 
and  cf  to  the  negative  pole  of  a  second  element  connected 
in  series  with  the  first  one. 

By  connecting  between  the  wires  a  and  c,  a  current  with 
a  difference  of  potential  of  220  volts  can  be  obtained. 

If,  therefore,  a  current  at  a  potential  of  110  volts  is  at 
disposal,  it  is  necessary  to  considerably  reduce  this  before 
it  is  suitable  for  carrying  out  electroanalytical  determina- 
tions. For  reducing  this  current  incandescent  lamps  are 
extremely  convenient.  An  ordinary  16-candle-power  110- 
volt  lamp  has  a  resistance  of  approximately  220  ohms. 
Therefore  when  it  is  connected  in  the  circuit  it  permits  the 
passage  of  a  current  having  the  strength  of  0.5  ampere. 


it:  c 


FIG.  53. 


FIG.  54. 


FIG.  55. 


The  simplest  arrangement  for  conducting  an  electrolytic 
determination  is  shown  in  Fig.  53.     In  this  case  the  electro- 
lytic cell  C  is  connected  in  series  with  a  single  lamp  *  L 
*  For  mounting  the  lamps  cheap  porcelain  sockets  are  very  convenient. 


REGULATING  THE  CURRENT-STRENGTH  AND  POTENTIAL.  109 

between  the  terminals  of  a  110-volt  circuit.  The  current 
through  the  cell  will  be  approximately  0.5  ampere.  In  Fig.  54 
the  circuit  contains  two  lamps  (16  C.  P.)  in  series,  and  the 
current  in  this  case  will  be  approximately  0.25  ampere. 
Two  lamps  in  parallel  will  give  a  current  of  one  ampere,  and 
three  lamps  in  series  a  current  of  approximately  0.16  ampere. 
By  a  proper  arrangement  of  lamps  in  the  circuit  almost  any 
desired  current-strength  can  be  obtained. 

This  sort  of  an  arrangement,  however,  while  perfectly  satis- 
factory for  depositing  most  metals  from  solutions  of  their  salts, 
is  not  suitable  when  the  separation  of  one  metal  from  several 
others  is  to  be  accomplished,  since  the  difference  of  potential 
between  the  electrodes  in  the  cell  is  dependent  on  the  con- 
ductivity of  the  solution  and  may  therefore  during  the  process 
of  electrolysis  increase  to  a  value  above  that  at  which  a  sep- 
aration can  be  effected. 

For  conducting  separations  of  different  metals  the  ar- 
rangement shown  in  Fig.  55  should  be  followed.  Two  or 
more  lamps  connected  in  parallel  are  placed  in  the  circuit, 
which  also  contains  a  variable  resistance  R  connected  in 
parallel  with  the  electrolytic  vessel  c.  By  properly  adjusting 
R  the  desired  difference  of  potential  between  the  electrodes 
can  be  maintained  throughout  the  electrolysis.  A  number 
of  other  modifications  of  this  method  are  possible  and  will 
undoubtedly  suggest  themselves. 


CHAPTER  XVI. 

ACCESSORY  APPARATUS. 

Electrodes. — The  electrodes  used  for  the  purposes  of 
electrochemical  analysis  are  in  nearly  all  cases  composed  of 
pure  platinum  or  of  a  platinum-iridium  alloy.  Excep- 
tions to  this  general  rule  are  the  mercury  cathodes 
which  are  sometimes  employed  for  determining  metals  in 
the  form  of  amalgams,  and  the  silver  anodes  used  in  the 
determination  of  the  halogens. 

The  advantages  of  the  platinum-iridium  alloy  (10% 
iridium)  over  pure  platinum  are  that  its  resistance  to  elec- 
trolytic action  is  as  great  as  that  of  pure  platinum  and  its 
greater  rigidity  and  elasticity  make  the  electrodes  composed 
of  it  less  liable  to  injury.  Electrodes  made  from  platinum- 
iridium  can  be  much  lighter  than  pure  platinum  electrodes 
of  the  same  dimensions. 

The  size  of  the  electrode  on  which  the  electrolytic  deposit 
is  precipitated  is  of  considerable  importance,  since  when  the 
exposed  surface  is  large  the  deposit  adheres  more  firmly  to 
it.  If  a  metal  separates  from  a  solution  in  a  dense  form,  as 
in  the  electrolysis  of  double  oxalates,  the  probability  of  the 
oxidation  of  the  metal  is  not  appreciably  increased  by  en- 
larging the  surface  of  the  cathode.  In  the  precipitation  of 
lead  and  manganese  peroxides  a  relatively  large  electrode 
surface  is  most  important.  It  is  not  practical,  therefore, 
to  employ  a  platinum  crucible  for  electrolytic  precipitation 
if  more  than  a  few  milligrams  are  to  be  separated;  not  only 

no 


ACCESSORY   APPARATUS.  Ill 

is  the  exposed  surface  too  small,  but  the  anode  and  cathode 
cannot  be  widely  enough  separated  to  facilitate  the  separation 
of  the  deposit  in  a  dense  form. 

The  nature  of  the  surface  of  the  electrodes  is  also  impor- 
tant, since  some  metals  separate  less  satisfactorily  on  ham- 
mered surfaces  than  on  those  which  have  been  spun  (or  rolled) 
and  polished.  In  some  cases,  as  in  the  precipitation  of  certain 
metals  and  in  the  determination  of  lead  as  peroxide,  the  firm 
adherence  of  the  precipitate  to  the  electrode  can  only  be  se- 
cured by  the  use  of  a  platinum  electrode  the  surface  of  which 
has  been  roughened  by  the  use  of  a  sand-blast. 

For  the  negative  electrode  (cathode)  the  author  uses  a  thin 
platinum  dish  having  the  form  shown  in  Fig.  56,  9  cm  in  diam- 
eter, 4.2  cm  in  depth, .  holding  about  250  cc  and  weighing 
from  35  to  37  grams.* 

Another  form  of  dish  electrode  is  shown  in  Fig.  57.  This 
is  the  form  recommended  by  v.  Klobukow,  and  differs  from 

*  Dishes  of  this  weight  must  be  composed  of  platinum-indium  to  be 
satisfactory.  The  relations  between  the  volume  of  the  contained  liquid 
and  the  available  electrode  surface  in  a  dish  having  exactly  the  size  and 
shape  given,  is  shown  in  the  following  table: 

Area  of  Electrode 
Volume  of  Liquid.  Surface,  in 

in  Cubic  Square 

Centimeters.  Centimeters. 

260  (full) 160 

250 .' 155 

200 130 

150 114 

120 100 

100 90 

The  dish  contains  150  cc  when  filled  to  within  1.7  cm  of  the  edge, 
and  120  cc  when  filled  to  within  2.2  cm. 

When  a  disk  anode,  4.5  cm  in  diameter,  is  used  with  this  dish  the 
most  uniform  distribution  of  the  current  is  obtained  by  adjusting  the 
disk  so  that  it  is  exactly  in  the  center  of  the  dish  and  about  2  cm  below 
the  edge. 


112 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


that  of  the  author  chiefly  by  having  a  lip  which  allows  liquids 
to  be  conveniently  poured  from  it. 


FIG.  56. 


FIG.  57. 


Since  dishes  which  have  become  rough,  scratched,  or  bent 
are   not   satisfactory   for   electrolytic   determinations,   it   is 
strongly  recommended  that  the  dishes  used 
as  electrodes  be  reserved  exclusively  for  their 
intended  purpose. 

As  anode,  the  author  uses  a  disk  of 
moderately  thick  sheet  platinum  (Fig.  58), 
about  4.5  cm  in  diameter,  which  is  fastened 
to  a  strong  platinum  wire.  The  disk  should 
have  a  number  of  good-sized  holes  in  it  to 
promote  the  circulation  of  the  liquid  and 
allow  the  ready  escape  of  the  gases  formed. 
The  author  has  also  used  an  anode  having 
.  the  form  of  the  platinum  dish  shown  in  Fig. 
56,,  50  mm  in  diameter  and  20  mm  in 
depth.  This  anode  is  supported  by  a  plati- 
num wire  attached  to  its  center  and  has 
five  openings  in  it.  It  is  particularly  suit- 
able for  the  determination  of  those  metals 
which  have  a  tendency  to  separate  in  a  spongy  form,  i.e., 
cadmium  and  bismuth. 

The  form  of  the  electrodes  used  at  the  Mansfeld  smelting- 
works  chiefly  for  the  determination  of  copper,  is  shown  in 


FIG.  58. 


ACCESSORY    APPARATUS.  113 

Figs.  59  to  62.     The  cathode*  may  be  either  a  platinum 


FIG.  59. 


FIG.  60. 


cylinder  (Fig.  59)  or  a  cone  (Fig.  60).  The  anodes  used  with 
these  are  shown  in  Fig.  61  and  Fig.  62  respect- 
ively. A  vertical  slit  in  the  cathodes  opposite 
the  supporting  wdre,  through  wrhich  the  wire  of 
the  anode  can  be  passed,  adds  to  the  con- 
venience in  adjusting  and  removing  the  cathodes. 
The  objection  to  this  form  of  electrode  is  that 
the  current  is  very  unequally  distributed  over 
the  surface  of  the  cathode,  the  current  density 
being  particularly  high  on  the  lower  edge, 
which  often  causes  the  deposited  metal  to  separate  there  in 


FIG.  61. 


*  An  important  advantage  of  this  form  of  cathode  is  that  it  can  be  used 
in  solutions  containing  a  precipitate.  When  the  dish  electrodes  are  used 
the  precipitate  settles  to  the  bottom  and  interferes  with  the  deposition 
of  the  metal. 


114 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


a  crystalline  or  spongy  condition.      The    current-density  is 
also  much  lower  on  the  outside  than  on  the  inside. 

A  form  of  electrode  recommended  by  Winkler  *  is  shown 
in  Fig.  63.     It  is  made  in  the  form  of  a  cylinder,  3.5  cm 


s 


FIG.  62. 


FIG.  63. 


in  diameter  and  5.5  cm  in  height,  from  platinum  wire  gauze 
(wire  0.12  mm  in  thickness,  250  meshes  per  sq.  cm).  The 
top  and  bottom  of  the  gauze  are  strengthened  by  a  rim  of 
sheet  platinum.  The  weight  of  the  electrode  is  from  13  to 
15  grams.  The  surface  area  can  be  approximately  calculated 
from  the  formula 


*Ber.  deutsch.  chem.  Ges.,  32,  2192  (1899). 


ACCESSORY   APPARATUS. 


115 


in  which  d  is  the  diameter  of  the  wire,  n  the  number  of  meshes 
per  sq.  cm,  I  the  circumference,  and  b  the  height  of  the 
cylinder. 

The  advantage  of  this  form  of  cathode  over  those  com- 
posed of  sheet  metal  is,  that  when  the  anode  shown  beside 
it  is  used,  the  current-density  on  the  inside  and  outside  of 
the  cylinder  is  practically  the  same.  Since,  in  electrolysis, 
the  metal  is  deposited  on  the  entire  circumference  of  the 
separate  wires,  there  is  much  less  tendency  for  the  precipitate 
to  scale  off,  and  as  a  consequence  satisfactory  deposits  can  be 
obtained  under  conditions  which  would  not  be  possible  with 
sheet-platinum  electrodes.  This  fact  permits  many  deter- 
minations to  be  carried  out  with  much  higher  current- 
densities  and  consequently  in  a  much  shorter  time. 

Another  form  of  electrode,  designed  by  Oettel  *  is  given 
in  Fig.  64.  The  cathode  is  a  sheet  of 
platinum  8  cm  high  and  5.5  cm  wide. 
The  anode  consists  of  two  parallel 
spirals  of  platinum  wire  attached 
to  a  forked  support,  and  is  so 
placed  that  the  spirals  are  at  equal 
distances  on  either  side  and  opposite 
the  middle  of  the  cathode.  With 
thisarrangement  the  current-density 
at  the  cathode  is  fairly  uniform. 

Stands.  —  For  holding  the  elec- 
trodes the  author  has  used  a  single 
standard  (Fig.  65)  having  a  metallic  ring,  to  which  three 
short  contact  points  of  platinum  are  riveted,  which  supports 
the  platinum  dish,  and  an  insulated  arm  (a),  which  carries 
the  anode.  An  objection  to  the  use  of  this  stand  is  that  the 
brass  rod  to  which  the  ring  and  arm  are  clamped  is  readily 

*Zeit,  f.  Elektrochemie,  2,  192  (1895-96). 


FIG   64 


116  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 


FlG.  65. 


FIG.  66 


FIG.  67. 


ACCESSORY    APPARATUS. 


117 


corroded  by  the  laboratory  vapors,  which  may  lead  to  an 
imperfect  electrical  connection.     The  stand  shown  in  Fig. 


FIG.  68. 

66  has  given  good  service  for  a  long  time.     The  ring  and  arm 
are  clamped  to  a  glass  rod  G,  and  n  is  connected  with  the 


FIG.  69. 


negative  and  p  with  the  positive  pole  of  the  source  of  current. 
The  anode  is  clamped  in  position  at  e.    If  a  cone  or  cylinder 


118  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

is  used  as  the  cathode,  two  arms  are  attached  to  the  glass 
rod,  as  shown  in  Fig.  67.  This  arrangement  is  particularly 
convenient  when  a  metal  is  precipitated  from  an  acid  solu- 
tion, since  by  lifting  the  standard  the  two  electrodes  can  be 
quickly  removed  from  the  electrolyte  and  plunged  into  a 
vessel  of  clean  water. 

Another  method  of  supporting  the  electrodes  is  to  have 
a  separate  standard  for  each  of  them  (Figs.  68,  69). 


FIG.  70. 


The  arrangement  shown  in  Fig.  70  has  been  used  by 
Herpin.  The  platinum  dish  P  supported  by  a  metal  tripod 
F  serves  as  the  cathode,  and  the  anode  is  a  spiral  of  platinum 


ACCESSORY    APPARATUS. 


wire  S  (shown  separately  in  Fig.  71).     The  dish  is  covered 
by  a  glass    funnel    which  prevents  loss 
of  the  solution  by  spirting. 

The  apparatus  used  by  Riche  is  shown 
in  Fig.  72.  The  cathode  (Fig.  73)  is  a 
cone  having  the  shape  of  a  crucible  open 
at  both  ends  and  provided  with  a  bail. 
It  contains  a  number  of  oblong  openings 
in  the  side  to  facilitate  the  circulation 
of  the  electrolyte.  The  cone  is  so  placed -in  a  platinum 


FIG.  71. 


FIG.  73. 


FIG.  72. 


crucible,  which  serves  as    anode,  that  the  distance  between 
them  is  from  2  to  4  mm. 


120 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


An  arrangement  for  carrying  on  several  similar  deter- 
minations simultaneously  has  been  described  by  v.  Malapert.* 
It  consists  of  a  wooden  frame  (Fig.  74)  at  the  top  of  which 

binding-posts  for  attaching 
the  electrodes  are  placed. 
Beakers  containing  the  solu- 
tions are  supported  by  the 
shelf  B.  Arrangements  of  this 
sort  are  very  convenient  when 
a  large  number  of  similar  de- 
terminations are  carried  out 
regularly,  as  is  often  the  case 
in  technical  laboratories. 

Where  several  similar  de- 
terminations are  made  at  the 

same  time  it  is  not  infrequently  the  practice  to  connect  the- 
various  cells  in  parallel  with  one  another,  and  to  assume 
that  the  current  distributes  itself  equally  between  them. 
This  assumption,  however,  is  usually  incorrect,  since  slight 
differences  in  the  resistance  of  the  cells,  due  to  differences  in 
the  concentration  and  composition  of  the  solution  and  to 
inequality  in  the  distances  between  the  electrodes,  will  cause 
marked  differences  in  the  strength  of  the  currents  passing 
through  them.  Because  of  this  inequality,  unsatisfactory 
results  will  be  obtained.  In  such  cases,  therefore,  the  cells 
should  always  be  connected  in  series,  so  that  the  exact 
current-strength  and  potential  of  each  cell  can  be  measured 
and  controlled. 

Many  separations  and  determinations  are  promoted  by 
heating  the  -electrolyte.  Great  care  should  be  exercised, 
however,  that  the  temperature  of  the  electrolyte  is  not  raised 


*  Zts.  f.  anal.  Ch.,  26,  56  (1887). 


ACCESSORY    APPARATUS. 


121 


to  the  boiling-point,  since  in*  this  case  the  precipitated  metal 
•will  be  loosened  from  the  electrode  and  its  quantitative  de- 
termination will  become  impossible.  To  insure  a  uniform 
heat,  which  is  also  essential,  a  thin  asbestos  board  may  be 
placed  under  the  dish  or  beaker,  and  the  source  of  heat  can 
be  either  the  burner  shown  in  Fig.  75  or  the  bottom  of  an 


FIG.  75. 

ordinary  Bunsen  burner  from  which  the  tube  has  been  re- 
moved so  that  the  gas  burns  in  a  small  luminous  flame. 

Engels,*  as  a  result  of  experiments  conducted  in  the 
Aachen  laboratory,  recommends  the  use  of  an  asbestos 
board  2  cm  below  the  dish  and  below 
this  an  ordinary  Bunsen  burner. 

The  arrangement  shown  in  Fig. 
76  can  also  be  used.  It  consists  of  a 
stand  made  by  bending  a  stout  cop- 
per wire  into  two  parallel  circles  con- 
nected by  an  upright.  The  smaller 
of  the  rings  supports  the  dish,  and  FlG-  76- 

the  heat  is  furnished  bv  the  small  flame  from  the  base  of 


*  Zts.  f.  Elektrochemie,  2,  413  (1895-96). 


122 


QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 


a  Bunsen  burner.  The  burner  is  covered  by  an  asbestos 
chimney,  which  is  made  by  moulding  wet  asbestos  board 
into  the  form  of  a  cylinder  with  openings  at  the  upper  and 
lower  edges  as  indicated.  The  top  of  the  cylinder  is  covered 
by  a  slightly  concave  sheet  of  asbestos. 


FIG.  77. 

A  universal  stand,  in  which  all  the  necessary  apparatus 
is  carried  by  a  single  vertical  rod,  has  been  described  by 
v.  Klobukow.*  A  sketch  of  this  is  shown  in  Fig.  77.  R  is 
the  dish  cathode,  E  the  anode ;  B  is  a  micro-burner  for  heating 
the  solution.  By  a  system  of  glass  and  rubber  tubing  pro- 

*  Journ.  f.  prak.  Chem.,  (2)  34,  539;  ibid.,  40,  121;  ibid.,  33,  473. 
See  also  Kriiger,  Elektrochem.  Zeit.,  3,  106  ff.  (1896). 


ACCESSORY    APPARATUS. 


123 


vided  with  pinch-cocks  the  contents  of  the  dish  can  be 
siphoned  off  into  the  beaker  G  and  fresh  water  for  washing, 
can  be  introduced  into  the  dish  from  F. 

Another  modification  of  v.  Klobukow  's  apparatus  is  shown, 
in  Fig.  78,  where  an  attachment  is  provided  for  imparting; 
a  slow  rotary  motion  to  the  anode  by  means  of  a  motor. 


FIG.  78. 

Through  the  courtesy  of  Professor  F.  A.  Gooch  of  Yale 
University  the  translator  is  able  to  include  a  description  of 
an  extremely  simple  and  practical  arrangement  for  electro- 
chemical analysis  which  possesses  the  additional  advantage 
that  it  dispenses  with  the  necessity  of  special  platinum 
dishes  or  electrodes.*  The  most  novel  feature  of  this 

*  Gooch  and  Medway,  Am.  Journ.  of  Science,  April,  1903. 


124 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


arrangement,  however,  is  that  by  its  use  the  time  required 
for  conducting  an  electroanalytical  determination  is  reduced 
to  from  10  to  30  minutes. 

The  appliance  (Fig.  79)  depends  upon  the  precipitation 

of  the  metal  upon  a  rapidly 
rotating  cathode.  The  cathode 
used  is  an  ordinary  platinum 
crucible  (20  cc  capacity),  which 
is  rotated  at  a  speed  of  600  to 
800  revolutions  a  minute  by  a 
small  electric  motor.  The  motor 
is  fastened  so  that  its  shaft  is 
vertical,  and  to  an  extension  of 
this  shaft  the  crucible  is  fixed 
by  pressing  it  over  a  rubber 
stopper  bored  centrally  and  fitted 
tightly  to  the  end  of  the  shaft. 
To  secure  electrical  connection 
between  crucible  and  shaft,  a 
narrow  strip  of  sheet-platinum 
is  soldered  to  the  shaft  and  then 
bent  upward  along  the  sides  of 

the  stopper  (A  in  Fig.  79),  thus  bringing  the  shaft  in  electrical 
contact  with  the  inside  of  the  crucible  when  the  latter  is 
pressed  over  the  stopper. 

The  solution  to  be  electrolysed  is  placed  in  a  beaker  upon 
a  small  adjustable  stand,  so  that  the  crucible  may  be  dipped 
into  the  liquid  to  any  desired  depth.  The  crucible  is  con- 
nected with  the  source  of  current  by  attaching  a  wire  to  one 
of  the  bearings  in  which  the  shaft  turns.  A  sheet  of  plati- 
num foil  suspended  from  the  edge  of  the  beaker  serves  as  the 
other  electrode. 

The  solution,  50  cc  in  volume,  was  placed  in  a  beaker 


FIG 


ACCESSORY    APPARATUS.  125 

having  a  total  capacity  of  about  150  cc,  and  the  height  of 
the  beaker  was  so  adjusted  that  the  liquid  covered  about 
two-thirds  of  the  crucible.  This  gave  a  cathode  surface 
having  an  area  of  about  30  sq.  cm. 

By  the  use  of  this  arrangement  quantities  of  copper  equal 
to  about  0.25  g  were  satisfactorily  precipitated  from  a 
sulphuric  acid  solution  with  a  current  of  XD100  =  10-13.3 
amperes  in  from  15  to  20  minutes;  0.19  g  of  silver  from  a 
solution  of  the  double  cyanide  with  a  current  of  ND100  = 
8.3-10  amperes  in  10  minutes;  and  0.17  g  of  nickel  from  a 
solution  containing  ammonium  sulphate  and  an  excess  of 
ammonia  with  a  current  of  ND100  =  1 1.7-13.3  amperes  in 
25  minutes. 

The  process  as  described  is  rapid,  exact,  and  very  simple, 
and  the  special  apparatus  required  is  inexpensive.  The 
speed  with  which  a  determination  can  be  carried  out  by 
this  method  would  seem  to  be  due  chiefly  to  the  complete 
and  constant  stirring  of  the  electrolyte,  by  which  fresh 
quantities  of  metal  ions  are  brought  to  the  surface  of  the 
cathode  for  discharge  and  deposition. 

It  will  probably  be  found  possible  to  apply  this  method 
with  success  to  most,  if  not  all,  separations  and  determi- 
nations. 

Vessels  for  Electrolysis. — For  the  special  purposes  of 
electrolysis,  in  addition  to  the  electrodes  and  dishes  de- 
scribed, a  large  number  of  other  forms  have  been  suggested. 
These  are  all  based  more  or  less  upon  the  same  principle. 
The  elbow  apparatus,  also  originated  by  v.  Klobukow,  de- 
serves mention.  In  this  the  gases  set  free  at  the  electrodes 
can  be  separately  collected  and  therefore  quantitatively 
determined.  The  apparatus  is  readily  understood  from 
Fig.  80.  The  corks,  which  are  paraffined,  carry  thick  plat- 
inum wires  to  which  the  round  flat  plates  which  serve  as 


126  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

electrodes  are  welded  at  angles  of  45°.  The  form  of  the 
electrodes  is,  of  course,  not  confined  to  any  particular  one; 
v.  Klobukow  also  suggests  round  fluted  platinum  foils,  wire 
spirals,  or  pointed  electrodes. 

In  case  the  anode  and  cathode  liquids  are  to  be  kept 
separate  by  a  porous  membrane,  v.  Klobukow  proposes  the 
.arrangement  shown  in  Fig.  81.  The  two  separate  arms 
have  close-fitting  ground  faces,  which  are  cemented  into  a 
brass  mounting.  A  tight  joint  is  obtained  by  a  hinge  and 
.screw. 


FIG.  80.  FIG.  81. 

An  electrolytic  apparatus,  depending  upon  another  prin- 
ciple and  serving  other  purposes,  which  nevertheless  might 
be  useful  for  quantitative  work,  is  described  by  Hofer.*  Fig. 
82  shows  two  electrode  chambers  of  glass  provided  with 
inlet  and  outlet  tubes  for  the  electrolyte,  which  is  conducted 
in  a  continuous  stream  through  the  apparatus.  There  is  also 
:an  escape  tube  for  the  gases  generated.  The  two  halves, 
between  which  parchment  paper  or  other  porous  diaphragm 
is  interposed,  are  fastened  together  by  a  firmly  cemented 
connection  provided  with  a  screw.  The  electrodes  have  the 
form  of  spirals  of  platinum  wire  0.8  mm  in  thickness,  or  of 
platinum  plates  attached  to  wires.  The  connecting 


*  Ber.  deutsch.  chem.  Ges.,  27,  461  (1894). 


ACCESSORY    APPARATUS. 


127 


wires  pass  through  the  gas-outlet  tubes,  and  in  case  the  gases 
.are  to  be  collected,  they  are  carried  on  through  T  tubes  placed 
at  the  top  and  made  tight  with  rubber  stoppers. 

The  liquid  to  be  electrolysed  is  contained  in  a  dropping- 
funnel,  the  tube  of  which  is  con- 
nected by  rubber  tubing  to  the 
lower  inlet  tube  of  one  section  of 
the  apparatus.  The  liquid  is  thus 
continually  brought  to  the  particu- 
lar electrode  and  is  made  to  circu- 
late through  the  cell  from  the  bot- 
tom to  the  top.  It  flows  out 
through  the  outlet  tube,  thence 
through  a  piece  of  rubber  tubing 
provided  with  a  screw  pinch-cock 
for  regulating  the  flow,  and  into  a 
vessel  placed  at  a  lower  level. 

This  piece  of  apparatus,  which  has  hitherto  been  used  only 
for  the  study  of  organic  decompositions,  might  perhaps  be 
suitable  for  the  quantitative  determination  of  gases. 


FIG.  82. 


CHAPTER  XVII. 

THE  ANALYTICAL  PROCESS. 

THE  complete  process  involved  in  the  quantitative  elec- 
trolytic determination  of  an  element  may  be  divided  into  a 
series  of  separate  operations,  as  follows: 

1.  Preparation  of  the  electrodes.  These  should,  of  course, 
be  scrupulously  clean.  For  scouring  and  polishing  them 
sea-sand  is  very  commonly  used.  This  should  be  of  good 
quality  and  free  from  sharp-cornered  grains,  or  otherwise 
the  surface  of  the  dishes  and  cylinders  will  become  scratched 
and  worthless  for  many  determinations.*  For  the  removal 
of  grease,  traces  of  which  on  the  cathode  are  extremely 
objectionable,  the  electrodes  may  be  heated  to  redness,  or 
cleaned  by  immersing  them  in  a  solution  of  chromic  acid 
in  concentrated  sulphuric  acid — the  so-called  "oxidising- 
mixture 7 ' — prepared  by  adding  powdered  potassium  bichro- 
mate to  ordinary  oil  of  vitriol.  After  washing  with  distilled 
water  and  thoroughly  drying  by  heating,  the  cathodes  are 
allowed  to  stand  in  a  desiccator  for  a  short  time  before  weigh- 
ing. The  surfaces  on  which  the  metals  are  to  be  deposited 
must  never  be  touched  with  the  fingers. 

*  The  translator  has  observed  that  platinum  cathodes  can  be  very  sat- 
isfactorily cleaned  with  a  commercial  product  known  as  "  Bon- Ami, "  which 
is  ordinarily  sold  for  cleaning  glassware.  Deeper  stains  can  be  removed 
successfully  with  "Sapolio,"  the  surface  being  afterwards  polished  with 
the  material  first  mentioned.  These  substances  are  applied  with  a  soft 
cloth  or  small  sponge,  the  platinum  surface  being  rubbed  gently  in  order 
to  avoid  the  useless  removal  of  metal. 

128 


THE  ANALYTICAL  PROCESS.  129 

2.  Preparation  of  the  solution.     This  is  conducted  accord- 
ing to  the  specific  directions  given  for  the  determination  to 
be  made,  and    to    obtain    satisfactory  results   these    direc- 
tions should  always  be  followed  as  closely  as  possible.     If  a 
dish  electrode  is  used,  the  mixture  of  the  various  salts  is 
best  conducted  in  a  beaker,  and  when  all  are  in  solution  the 
liquid  is  transferred  to  the  dish,  where  it  is  finally  diluted  to 
the  proper  volume.     If  the  electrolysis  is  to  be  conducted 
at  an  elevated  temperature,  the  solution  should  be  wanned 
to  the  proper  point  before  starting  the  current. 

3.  Attachment  of  electrodes  to  circuit.     This  is  carried  out 
in  a   manner  dependent   upon   the   form  of  electrode   em- 
ployed -  and  the  kind   of   standard   used.     The   anode   and 
cathode  should  be  so  adjusted  that  the   current-density  at 
the  electrode  on  which  the  precipitation  is  to  take  place  will 
be  as  uniform  as  possible.     If  the  anode  shown  in  Fig.  61  is 
used,  it  should  extend  to  the  bottom  of  the  vessel  in  which 
the  electrolysis  is  conducted.     In  general  the  conditions  in 
the  circuit  should  be  such  that  sufficient  resistance  is  present 
to  prevent  an  abnormally  high  current-strength  or  difference 
of  potential  when  the  electrolysis  is  started,  since  this  would 
lead  to  unsatisfactory  results  in  many  separations.     It   is 
best,  therefore,  to  have  sufficient  resistance  in  the  connected  1 
rheostat  so  that  when  the  circuit  with  the  source  of  current 
is  closed,  a  current  no  greater  than  the  maximum  required 
for  the  given  electrolysis  will  pass  through  the  circuit. 

It  is  also  important  that  all  metallic  contacts  in  the  cir- 
cuit should  be  clean  and  rigid,  since  otherwise  the  current 
may  become  weakened  or  interrupted.  This  applies  to  the 
battery  connections,  to  the  connections  between  the  wires 
and  the  various  instruments  in  the  circuit,  and  to  the  con- 
nections between  the  electrodes  and  the  supporting  standards. 

4.  The    electrolysis.      The    circuit    with    the    source    of 


130          QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

current  having  been  completed,  the  resistance  is  adjusted 
so  that  the  current-strength  and  difference  of  potential 
between  the  electrodes  correspond  with  those  given  in  the 
directions.  Since  the  relative  values  of  these  two  factors 
:are  dependent  on  the  resistance  of  the  cell — which  in  turn 
depends  upon  the  actual  conductivity  of  the  solution,  the 
size  and  shape  of  the  electrodes,  and  the  distance  by  which 
they  are  separated — it  will  frequently  be  found  that  the 
current  cannot  be  so  regulated  that  the  values  of  both  of 
these  factors  will  correspond  with  those  described  by  some 
other  experimenter.  In  such  a  case  it  is  in  general  best  to 
bring  the  potential  to  the  desired  value  and  allow  the  current- 
strength  to  adjust  itself  to  this  condition.  This  is  especially 
true  in  the  case  of  the  electrolytic  separation  of  a  metal  from 
others  contained  in  the  same  solution,  where  the  potential- 
difference  is  usually  the  factor  of  chief  importance. 

A  very  convenient  method  for  determining  the  com- 
pletion of  a  precipitation  consists  in  placing  a  small  strip  of 
bright  platinum  foil  in  contact  with  the  cathode.  If  after 
some  time  no  deposit  is  formed  on  it,  it  is  safe  to  assume 
that  the  electrolysis  is  completed.  If  a  deposit  is  formed 
it  can  be  quickly  removed  by  placing  the  foil  in  contact 
with  the  anode  for  a  few  moments.  This  method  is  of  course 
unsuited  for  metals  which  when  deposited  closely  resemble 
platinum. 

In  order,  during  electrolysis,  to  prevent  the  loss  of  a  por- 
tion of  the  solution  in  the  form  of  small  drops  thrown  up- 
ward by  the  escape  of  gas  bubbles,  the  electrolytic  vessel 
should  be  covered.  When  a  dish  electrode  is  used  the  cover 
can  consist  of  a  watch-glass  cut  into  two  equal  halves  by  the 
use  of  a  diamond,  or  of  a  watch-glass  perforated  by  a  single 
small  opening.  The  hole  in  the  watch-glass  can  be  made 
with  the  point  of  a  file  moistened  with  a  solution  of  camphor 


THE  ANALYTICAL  PROCESS.  131 

in  turpentine.     The  cover  also  greatly  reduces  the  evapora- 
tion of  water  from  warm  electrolytes. 

5.  The  removal  of  the  solution,  washing  and  drying  the 
electrode.  Many  determinations  require  the  removal  of  the 
solution  from  the  cathode  without  interrupting  the  current, 
since  otherwise  the  deposited  metal  would  again  pass  into 
the  solution.  When  dish  electrodes  are  used  they  can  be 
placed  on  triangles  of  platinum  wire  on  the  bottom  of  good- 
sized  beakers,  and  when  the  electrolysis  is  completed  the 
solutions  can  be  displaced  by  a  stream  of  fresh  water ;  or  the 
solution  in  the  dish,  supported  on  an  ordinary  stand,  can  be 
siphoned  off,  the  deposited  metal  being  washed  during  the 
process  by  a  stream  of  water  from  a  wash-bottle.  For  most 
purposes,  however,  the  results  obtained  are  sufficiently 
accurate  when  the  current  is  stopped  and  the  contents  of 
the  dish  are  immediately  poured  out,  the  precipitated  metal 
being  quickly  washed  with  pure  water.  When  the  cylin- 
drical cathodes  are  used,  the  electrode,  still  attached  to  the 
stand,  can  be  lifted  quickly  from  the  solution  and  plunged 
into  a  vessel  of  clean  water.  After  a  thorough  washing  with 
water,  the  electrodes  are  washed  three  times  with  about  5  cc 
of  pure  absolute  alcohol,  dried  for  about  five  minutes  in  an 
air-bath  at  about  70°  to  90°,  allowed  to  cool  thoroughly  in 
a  desiccator,  and  weighed. 


CHAPTER  XVIII. 

ARRANGEMENTS  FOR  ANALYSIS. 

THE  question  as  to  the  most  suitable  equipment  for 
electroanalytical  experiments  does  not  permit  of  a  general 
answer,  owing  to  the  numerous  details,  such  as  the  location 
and  construction  of  the  building,  the  arrangement  of  rooms, 
etc.,  upon  which  it  depends.  Even  if  the  use  of  accumula- 
tors in  combination  with  a  dynamo  is  decided  upon  as  the 
most  practical  source  of  current,  the  details  of  the  equipment 
can  be  described  only  from  a  certain  point  of  view,  according 
to  the  specific  requirements.  The  laboratory  at  Aachen  has 
followed  the  development  of  quantitative  electrolysis  almost 
from  the  beginning,  and  starting  with  a  small  and  simple 
equipment  has  gradually  acquired  a  most  elaborate  one. 

Three  equipments  will  therefore  be  described:  first,  a 
simple  and  practical  arrangement  for  use  where  the  require- 
ments are  limited ;  second,  the  former  electrolytic  equipment 
of  the  Aachen  Institute  of  Technology ;  and  third,  the  present 
equipment  of  the  same  institution. 

Kriiger  *  has  published  a  general  review  on  the  equip- 
ment of  electrolytic  laboratories  which  contains  many  val- 
uable suggestions.  The  arrangements  of  certain  other  lab- 
oratories have  also  been  described  by  Nissenson.f  The 
choice  of  special  apparatus  depends  so  much  on  individual 

*  Elektrochem.  Zeit.,  2,  73,  104,  129,  174,  207,  251;  ibid.,  7,  76,  129. 
t  Zeit.  Elektrochem.,  6,  221. 

132 


ARRANGEMENTS  FOR  ANALYSIS. 


133 


taste,  that  exact  directions  are  practically  impossible.  In- 
deed the  practical  advantages  of  the  set  of  instruments  recom- 
mended by  Kriiger  have  not  been  confirmed  by  their  use  in 
the  Aachen  laboratory. 

Simple  Arrangement  for  Electrolysis. 

The  equipment  needed  for  carrying  out  a  single  elec- 
trolytic experiment,  when  a  constant  source  of  current  is 
at  hand,  is  an  extremely  simple  one.  A 
standard,  a  pair  of  electrodes  and  instru- 
ments for  measuring  the  current-strength 
and  potential-difference  are  all  that  are 
required.  The  manner  in  which  the 
various  pieces  of  apparatus  are  con- 
nected is  shown  in  Fig.  83.  The  am- 
peremeter (A)  is  connected  in  series  in 
the  circuit  with  the  cell,  and  the  volt- 
meter circuit  is  attached  directly  to 
points  on  the  standard  in  metallic  con- 
tact with  the  cathode  and  anode.  The 
voltmeter  is  represented  by  V,  the  FIG.  83. 

variable  resistance  for  controlling  the  current  by  R,  and  the 
source  of  current  by  S. 

Since,  however,  it  is  often  desirable  to  conduct  several 
experiments  simultaneously,  an  arrangement  for  accom- 
plishing this  will  be  described  which  has  the  advantage  that 
it  can  be  constructed  by  any  one  wishing  to  carry  out  elec- 
trolytic determinations. 

The  chief  requirement  is  that  it  shall  be  possible  at  any 
time  to  measure  the  current-strength  and  potential  of  each 
separate  cell,  which  can  be  accomplished,  with  the  use  of 
one  amperemeter  and  one  voltmeter,  in  the  following  manner: 


134 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


Two   blocks   carrying   six  binding-posts   each   are   con- 
structed as  shown  in  Fig.  84.     These  consist  of  a  wooden 


FIG.  84. 

block  supporting  a  copper  plate  through  which  are 
drilled  six  holes  having  a  diameter  sufficient  to  permit  the 
screws,  but  not  the  bases,  of  the  binding-posts  to  pass. 
The  binding-posts  are  screwed  through  these  holes  into 
the  wooden  block  beneath  until  a  close  metallic  contact 
between  the  posts  and  the  copper  plate  is  obtained.  One 
of  these  blocks  (Fig.  85,  A)  is  connected  by  one  of  its 


y  ^ 

-s   vc 

*   ^c 

*= 

;/ 

wr 

9      9 

9      o 

W3 

9      9 

W4 

0           O 

J       T 

^_. 

/I 

/ 

FIG.  85. 


posts  with  the  negative,  the  other  (B)  with  the    positive 
pole    of  the  source    of  current.     Another    wooden   block, 


ARRANGEMENTS  FOR  ANALYSIS.  135 

/,  is  placed  in  front  of  the  block  connected  with  the  nega- 
tive pole.  This  block  has  a  number  of  holes  bored  in  its 
upper  side,  and  into  these  holes  are  set  inverted  thimbles 
filled  with  mercury.  The  arrangement  of  these  holes  is 
shown  in  Fig.  86,  and  those  (7,  2,  3,  and  4)  along  one  edge 
are  connected  with  corresponding  binding-posts  on  the  block 
A  by  stout  copper  wires  which  dip 
into  the  mercury.  The  mercury- 
cups  1' ,  2' ,  3',  4f  are  connected  by 
wires  to  one  terminal  of  the  rheo- 
stats wl}  w2,  Wj,  w4,  respectively,  and 
the  other  terminals  of  the  rheo- 
stats are  connected  by  wires  with  /T^)  ^-^  (^\ 
posts  on  the  block  B,  The  cells  in  @  ©  ^ 
which  the  various  electrolytic  deter- 


00000 


minations  are  carried  out  are  in-  FIG.  86. 

serted  in  the  circuits  between  the  rheostats  and  the  positive 
pole  (Clt  C2,  C3,  C4).  The  circuit  through  cell  Ctis  completed 
by  laying  a  bridge  made  of  copper  wire  between  the  mercury- 
cups  a  and  1' ',  and  the  current  then  flows  from  B,  through  Clf 
through  u\j  from  1'  to  a,  and  through  the  amperemeter  to 
A.  The  current-strength  can  be  read  from  the  ampere- 
meter and  regulated  by  the  variable  resistance  wv.  In  order 
to  proceed  with  the  second  experiment  (C2)  the  cell  is  con- 
nected as  shown,  between  B  and  TF2.  A  second  copper-wire 
bridge  *  is  now  laid  between  mercury-cups  1  and  lf  and  the 
first  bridge  between  a  and  1'  is  removed  and  laid  between  a 

*  When  the  internal  resistance  of  the  amperemeter  is  appreciable, 
which  is  not  the  case  with  the  standard  Weston  instruments,  the  bridge 
used  to  connect  the  mercury-cups  1  and  lr,  2  and  2',  3  and  3'y  etc.,  must 
have  a  resistance  exactly  equal  to  that  of  the  amperemeter.  Otherwise 
the  current  as  measured  with  the  amperemeter  will  be  less  than  that 
which  passes  through  the  cell  when  the  amperemeter  is  removed  from 
that  circuit. 


136  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

and  #'.  The  current  through  C2  now  passes  through  the 
amperemeter  and  can  be  observed  and  regulated.  In  an 
exactly  similar  manner  the  cells  C3  and  C4  are  connected 
with  the  main  circuit.  In  order  at  any  time  to  observe 
the  current  passing  through  any  given  cell  the  correspond- 
ing mercury-cup  on  the  lower  edge  of  the  board  /  is  con- 
nected with  a  and  the  bridge  between  the  two  opposite  cups 
on  /  is  removed.  In  this  way  it  is  possible  to  measure  each 
of  the  separate  currents  without  causing  any  interruption. 
All  the  connections  in  the  main  circuits,  including  the  con- 
nections of  the  amperemeter,  should  be  made  by  stout 
copper  wires. 

For  measuring  the  differences  of  potential  between  the 
separate  electrodes,  another  wooden  block,  Fig.  87,  is  pre- 
pared. This  block  contains  four  mercury-cups  (S1}  S2,  S3,  S4) 

arranged  on  the  arc  of  a  circle  along 
one  edge  and  one  cup  on  the  edge 
opposite.  The  cups  Sl}  S2,  S3,  S4  are 
connected  with  the  terminals  of 
the  rheostats  wly  iv2,  w3,  w4,  as  shown 
in  Fig.  85,  and  the  cup  S  is  con- 
nected with  the  negative  binding- 
post  of  the  voltmeter,  the  positive 
post  of  this  instrument  being  con- 
FIG.  87. .  nected  directly  with  one  of  the 

binding-posts  on  the  block  B.  By  placing  a  copper- wire  bridge 
between  the  cups  S-Slt  S-S2,  S-S3,  or  S-S4,  the  difference  of 
potential  between  the  electrodes  in  the  cells  Clt  C2,  C3,  or  C4, 
respectively,  can  be  read  with  the  voltmeter.  The  simul- 
taneous measurement  of  the  potential  of  two  or  more  cells  is 
of  course  out  of  the  question.  By  increasing  the  number  of 
binding-posts  on  the  blocks  A  and  B,  and  the  number  of 
mercury-cups  in  the  blocks  /  and  //,  the  number  of  sep- 


0©®© 


© 


ARRANGEMENTS  FOR  ANALYSIS.  137 

arate  determinations  which  can  be  conducted  at  one  and  the 
same  time  can  be  increased  to  any  reasonable  number. 

This  simple  appliance,  the  principles  of  which  recur  in 
the  following  pages,  can  be  prepared  by  any  one  from  the 
simplest  materials,  making  it  very  suitable  for  the  use  of 
students  who  by  working  with  it  become  acquainted  with 
the  methods  of  making  connections  and  the  manipulation 
of  more  elaborate  apparatus. 

Former  Equipment  of  the  Electrochemical  Institute  at  Aachen. 

This  system  was  based  on  the  employment  of  a  dynamo 
which,  running  at  a  speed  of  1000  revolutions  per  minute, 
furnished  a  current  at  a  potential  of  10  volts.  The  current 
from  the  dynamo  was  used  either  directly  or  for  charging 
accumulators. 

When  the  current  from  the  dynamo  was  used  directly 
for  electrolytic  purposes,  the  instrument  described  on  p.  106 
was  employed  for  reducing  and  regulating  the  current. 

In  general  the  current  from  the  dynamo  was  used  to 
charge  four  accumulators,  connected  in  series,  and  the  cur- 
rent from  these  having  a  potential  of  8  volts  was  carried 
by  suitable  conductors  to  the  work-bench  used  for  elec- 
trolytic experiments.  The  connections  of  the  electrolytic 
cells  to  the  positive  conductor  were  effected  by  means  of  the 
six  binding-posts  marked  1,  2,  3,  4,  5,  6  (Plate  I,  Fig.  1). 
For  connecting  the  electrolytic  cells  with  the  negative 
conductor,  six  wooden  blocks  carrying  binding-posts  and 
mercury-cups  were  employed. 

The  arrangement  of  the  posts  and  cups  on  these  blocks  is 
dia grammatically  shown  in  Figs.  88  and  89  (one-fourth  actual 
size).  The  four  mercury-cups,  1,  2,  3,  4,  are  in  metallic  con- 
tact with  the  four  binding-posts  marked  K  in  the  diagram, 
cups  5.  and  7  are  both  connected  with  the  negative  conductor, 


138 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


and  cup  6  is  connected  with  one  of  the  wires  leading  to  the 
amperemeter.  (The  position  of  these  six  blocks  with  respect 
to  the  circuit  is  shown  in  Plate  I,  Fig.  1,  where  they  are 


©    ©    © 


®    ® 

©  ©  c 

P  ^ 

D 

©  © 
c 

)  « 

) 

FIG.  88. 


FIG.  89. 


denoted  by  a  —  sign,  and  in  Plate  I,  Fig.  2,  where  they  are 
denoted  by  the  letter  N.) 

In  order  to  carry  out  an  electrolysis  at  a  given  current- 
strength  a  wire  was  carried  from  one  of  the  positive  binding- 
posts  and  attached  to  the  anode  of  the  cell,  and  the  cathode 
orthe  cell  was  connected  by  wires  to  one  of  the  binding-posts 
on  the  block  connected  with  the  negative  conductor.  A 
rheostat  was  inserted  in  the  circuit  between  the  cathode  and 
the  negative  binding-post,  and  by  means  of  this  a  moderately 
high  resistance  of,  say,  60  ohms  was  introduced  into  the 
cell  circuit.  The  object  of  introducing  this  high  resistance 
at  the  start  was  to  prevent  the  current-strength  from  attain- 
ing an  undesirably  high  value  when  the  circuit  was  com- 
pleted. To  complete  the  circuit  it  was  only  necessary  to  lay 
a  copper  bridge  between  the  mercury-cups  4  and  6.  The 
current  would  then  pass  from  the  positive  conductor,  through 
the  cell,  through  the  rheostat,  to  the  mercury-cup  on  the 
block,  and  through  the  amperemeter  to  the  negative  con- 
ductor. (These  connections  are  shown  in  Plate  I,  Fig.  1, 
position  5.) 

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ARRANGEMENTS    FOR    ANALYSIS. 


139 


meter  would  show  only  a  very  small  current,  which  was 
increased  to  the  desired  value  by  reducing  the  resistance 
in  the  rheostat.  When  this  was  done,  a  bridge  was  inserted 
between  the  mercury-cups  4  and  7  and  that  between  4,  and 
6  was  removed.  By  the  latter  operation  the  amperemeter 
was  disconnected  from  the  circuit  (position  3  in  the  diagram). 
In  order  to  observe  the  current-strength  at  any  time  during 
the  electrolysis,  a  bridge  was  laid  between  cups  4  and  6 
and  the  one  between  4,  and  7  was  removed.  It  was,  there- 
fore, possible  to  measure  the  current-strength  at  any  time 
without  interrupting  the  current  through  the  cell. 

The  amperemeter  was  especially  constructed  for  the 
laboratory  by  the  firm  of  Hartmann  &  Braun  (Bocken- 
heim-Frankfurt  a.  M.).  It  had  two  scales  and  two  pointers 
(one  on  each  side),  and  the  scales  had  a  radius  of  16  cm. 
This  instrument  permitted  the  measurement  of  currents 
up  to  2  amperes  in  decimals  of  0.05  ampere,  and  was  provided 
with  a  shunt-resistance  which  could  be  connected  in  parallel 
with  it,  whereby  the  range  of  measurement  could  be  increased 
tenfold. 

The  resistance  of  the  amperemeter  was  0.32  ohm,  and 
in  order  that  the  current-strength  should  remain  perfectly 


Resistance-roll  having  a  resistance 
equal  to  that  of  the  amperemeter. 


FIG.  90. 


constant  when  it  was  removed  from  the  circuit,  the  bridge 
(Fig.  90)  which  was  substituted  for  it  between  the  mercury- 
cups  4  and  7  was  not  of  simple  construction,  but  contained 


140  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

a  roll  having  a  resistance  exactly  equal  to  that  of  the  am- 
peremeter. Since  the  resistance  of  the  bridge  and  ampere- 
meter was  equal,  a  current  corresponding  to  the  one  meas- 
ured flowed  through  the  cell  when  the  amperemeter  was 
removed  from  the  circuit. 

For  measuring  the  difference  of  potential  between  the 
electrodes  of  a  cell  a  second  series  of  blocks  connected  with 
a  voltmeter  was  employed  (M.  S.,  Plate  I,  Fig.  2).  These 
blocks  contained  five  mercury-cups,  one  at  the  center  and 
four  others  at  equal  distances  from  this.  The  one  in  the 
center  was  connected  with  the  voltmeter  circuit,  the  other 
four  were  each  in  contact  with  a  separate  binding-post. 
For  measuring  the  potential  a  wire  was  connected  with  one 
of  these  binding-posts  and  the  other  end  of  the  wire  was 
attached  to  the  stand  supporting  the  cathode.  By  placing 
a  copper  bridge  between  the  mercury-cup  in  contact  with 
the  given  post  and  the  cup  in  the  center  of  the  block  the 
cathode  was  brought  into  electrical  connection  with  the 
voltmeter,  and  since  the  voltmeter  was  in  turn  connected 
with  the  positive  conductor,  the  potential  which  it  registered 
was  that  existing  between  the  electrodes  in  the  cell.  This 
connection  is  shown  in  Plate  I,  Fig.  2,  to  the  left  of  the 
diagram. 

As  shown  in  Plate  I,  twenty-four  separate  electrolytic 
experiments  could  be  conducted  simultaneously. 

A  view  of  the  author's  private  laboratory,  showing  the 
former  equipment,  is  given  in  Plate  II.  In  this  laboratory 
special  circuits  were  provided  for  supplying  the  direct  current 
from  the  dynamo,  as  well  as  that  from  eight  accumulators. 
The  wire-gauze  resistance  described  on  p.  106  was  used  to 
reduce  the  current  from  the  dynamo  when  this  was  employed 
directly  or  for  charging  the  accumulators.  The  circuit  from 
the  dynamo  and  the  circuit  from  the  accumulators  passed 


ARRANGEMENTS    FOR    ANALYSIS.  141 

from  the  private  laboratory  to  the  work-benches  in  the 
laboratory  of  instruction.  One  amperemeter  showed  the 
current  which  was  being  used  in  the  laboratory  of  instruc- 
tion, and  another  served  to  measure  the  current  used  in 
charging  the  accumulators. 

Present  Equipment  of  the    Electrochemical   Institute  at 

Aachen. 

In  the  former  equipment  of  the  Electrochemical  Institute 
at  Aachen  the  electric  current  was  supplied  by  a  generating 
plant  on  the  premises.  In  designing  the  present  equipment 
it  was  considered  desirable  to  be  as  independent  of  such  a 
plant  as  possible,  since  small  isolated  plants  are  uneconom- 
ical and  are  not  always  ready  for  use. 

To  avoid  the  maintenance  of  a  private  electric  generating 
plant  it  was  decided,  therefore,  to  take  the  current  from  the 
cables  of  the  municipal  electric  system  of  the  city  of  Aachen. 

The  Aachen  Electrical  Works  supply  the  direct  current 
by  a  three-wire  circuit  at  a  potential  of  about  108  volts  be- 
tween the  middle  wire  and  an  outside  wire,  and  a  potential 
of  about  216  volts  between  the  two  outside  wires.  It  is 
therefore  necessary,  for  the  purposes  of  electrolysis,  to  reduce 
this  high  potential  in  some  suitable  manner  to  the  IOWT  poten- 
tial required  for  experiment.  This  is  accomplished  by  the 
use  of  a  rotary  transformer,  which  is  efficient,  practically 
noiseless,  convenient,  and  compact. 

It  is  also  desirable  to  have  the  high  potential  current 
available  for  other  purposes. 

Before  proceeding  to  the  description  of  the  plant  installed 
by  the  firm  of  Schuckert  &  Co.,  proprietors  of  the  Aachen 
Electrical  Works,  the  nature  of  the  different  experiments 
carried  out  in  the  laboratory  will  be  mentioned  briefly  in 
order  that  what  follows  may  be  more  readily  understood. 


142  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

1.  Experiments  with  Low  Potentials. — The  experiments  with 
low  potentials  are  confined  chiefly  to  the  electrolytic  analysis 
of  solutions  of  metallic  salts.     Experiments  on  the  electrolytic 
precipitation  of  metals  on  a  large  scale  are  also  carried  out 
as  an  introduction  to  the  study  of  electrometallurgy. 

2.  Experiments  with  High  Potentials. — The  high-potential 
current  is  employed  chiefly  for  experiments  with  the  electric 
furnace,   for  the   decomposition   of  fused  electrolytes,   and 
for  the  decomposition  of  gases  and  other  bodies  having  a 
high  resistance.     For  producing  the  Davy  arc  a  potential 
of  about  45  volts  is  required. 

In  addition  to  the  above,  the  current  is  also  used  in  an 
electric  projection  lantern  and  for  a  number  of  arc  and  incan- 
descent lights. 

The  distribution  of  the  currents  to  the  various  rooms, 
and  the  control  of  the  transformer,  is  effected  from  a  central 
switchboard  located  in  the  author's  private  laboratory.  By 
this  arrangement  the  switchboard  is  placed  under  competent 
supervision  and  a  general  oversight  of  the  entire  plant  is  possible. 

From  the  central  switchboard  circuits  are  carried  to  the 
following  places : 

1.  Private  laboratory. 

2.  Large  lecture-room. 

3.  Laboratory  for  electrochemical  analysis. 

4.  Laboratory  for  experiments  on  a  large  scale  with  high 
and  low  potentials. 

The  circuits  running  to  the  different  rooms  are  distin- 
guished, according  to  the  intended  purposes  of  the  current 
which  they  carry,  as 

a.  Lighting  circuits, 

b.  High-potential  circuits, 

c.  Low-potential  circuits, 
and  are  entirely  independent  of  one  another. 


ARRANGEMENTS    FOR   ANALYSIS.  143 

The  lighting  circuits  run  to  the  private  laboratory  and 
to  the  large  lecture-room. 

The  high-potential  circuits  are  carried  to  the  private 
laboratory,  the  large  lecture-room,  and  to  the  laboratory  for 
experiments  with  high  and  low  potentials. 

In  addition  to  these  circuits  there  is  one  for  charging 
the  accumulators  and  another  for  running  the  transformer. 

The  switches,  rheostats,  safety-fuses,  and  measuring  in- 
struments for  the  different  circuits  are  placed  on  the  central 
switchboard 

1.  PRIVATE  LABORATORY. 

The  private  laboratory  contains  the  central  switchboard 
and  the  battery  of  accumulators.  A  photographic  view  of 
the  interior  is  given  in  Plate  III.  In  the  center  can  be  seen 
the  switchboard  upon  which  the  various  instruments  are 
mounted;  to  the  left  is  the  glass  hood  containing,  in  the 
bottom,  the  battery  of  accumulators.  Along  the  wall  on  the 
right  are  two  work-benches,  one  for  electroanalytical  work 
with  low  potentials  and  small  currents,  the  other  for  experi- 
ments with  high  potentials  and  large  currents. 

The  arrangement  for  electroanalysis  is  the  following:  At 
the  back  of  the  bench  by  the  window  is  a  slanting  wooden 
frame,  on  the  face  of  which  are  f;  stened  the  switches  and 
binding-posts,  while  the  connecting  wires  are  attached  to  the 
back.  There  are  altogether  five  work-places  on  this  bench, 
at  each  of  which  two  analyses  can  be  performed  simultane- 
ously, so  that  in  all  ten  experiments  can  be  carried  on  at  the 
same  time. 

The  installation  of  these  work-places,  as  well  as  those  of 
the  second  work-bench,  is  in  accordance  with  the  scheme 
for  current  distribution  shown  in  Plate  V. 

Each  work-place  is  connected  in  parallel  to  the  positive 


144  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

and  negative  conductors,  which  run  through  the  work- 
bench. 

The  current  for  every  analysis  can  be  independently  varied 
by  means  of  the  regulating  resistance  at  the  w^ork-place.  A 
single  amperemeter,  which  can  be  thrown  into  the  circuit 
of  any  analysis  by  means  of  a  switch  placed  at  each  work- 
place, serves  for  measuring  the  current-strength.  When  the 
amperemeter  is  cut  out,  its  place  is  taken  by  a  resistance,  in 
order  that  the  current-strength  will  not  be  altered  (see  p.  135). 

The  measurement  of  the  potential  is  carried  out  in  a  similar 
manner  by  a  single  voltmeter,  which  can  at  will  be  switched 
into  the  circuit  of  any  analysis  in  progress. 

A  lead  safety  fuse  is  inserted  in  the  circuit  of  each  of  the 
ten  branches  to  guard  against  the  possibility  of  too  great 
current-strength . 

The  connections  of  the  electrolytic  apparatus  to  the  small 
switchboards  of  the  work-bench  are  made  with  very  flexible 
rubber-insulated  copper  conductors,  the  ends  of  which  are 
provided  with  small  copper  links  to  allow  them  to  be  more 
conveniently  attached  to  the  apparatus. 

For  conducting  experiments  with  large  currents  of  high 
or  low  potential,  two  cases  furnished  with  locks  are  affixed  to 
the  second  work-bench.  That  for  low  potential  contains  two 
plates  which  carry  a  number  of  binding-posts,  thus  allowing 
several  different  pieces  of  apparatus  to  be  connected  at  the 
same  time. 

The  case  for  high  potential  contains  three  plates,  connected 
with  the  two  outside  conductors  and  the  middle  conductor 
of  the  three-wire  system  so  that  a  maximum  potential  of 
about  216  volts  is  obtainable.  These  plates  also  carry  several 
binding-posts,  which  permit  the  use  of  several  pieces  of  appa- 
ratus at  one  time. 

The  two  accumulator  batteries  contain  four  cells  each. 


ARRANGEMENTS  FOR  ANALYSIS.  145 

One  battery  with  the  cells  connected  in  series  requires  a 
charging  current  of  90  amperes;  the  other,  similarly  con- 
nected, requires  25  amperes. 

The  batteries  are  charged  from  the  transformer. 

The  small  battery  furnishes  current  to  the  private  labora- 
tory only,  while  the  large  one  supplies  the  rest  of  the  plant. 
Each  of  the  batteries  is  provided  with  a  cell  switchboard  for 
four  cells,  so  that  by  cutting  out  separate  cells  the  potential 
of  the  current  may  be  reduced  and  the  use  of  high  external 
resistances  avoided. 

To  prevent  the  direction  of  the  current  becoming  reversed 
during  the  process  of  charging,  each  battery  circuit  is  pro- 
vided with  an  automatic  cut-out. 

The  potential  of  the  separate  cells  is  measured  by  a  special 
voltmeter,  having  contact  plugs  which  allow  the  potential 
of  each  cell  to  be  independently  measured  at  the  cell  switch- 
board. 

For  the  measurement  of  the  battery  potential  and  the 
strength  of  the  charging  and  discharging  currents  a  special 
voltmeter  and  amperemeter  are  provided.  Further,  that  the 
operation  of  charging  and  discharging  may  be  more  closely 
observed,  indicators  for  showing  the  direction  of  the  current 
are  attached  to  the  several  circuits. 

2.  LARGE  LECTURE-ROOM. 

The  installation  of  the  large  lecture-room  is  especially 
intended  for  the  performance  of  lecture  experiments  which 
comprise  the  demonstration  of  electrolysis,  the  decomposi- 
tion of  gases  and  liquids  by  the  Davy  arc,  and  fusion  ex- 
periments. 

Besides  this,  provision  is  made  for  running  an  electric 
projecting  lantern,  as  well  as  a  number  of  incandescent  and 
arc  lamps. 


146       QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS. 

3.  LABORATORY  FOR  THE  ELECTRO  ANALYSIS  OF  METALS. 

In  this  room  the  transformer  is  placed.  It  also  contains 
a  large  table  having  ten  work-places  for  carrying  out  electro- 
analytical  experiments  with  low  potentials.  (Cf.  Plate  IV.) 

The  transformer  will  next  be  described.  This  consists 
of  a  combination  of  two  direct-current  dynamos  with  their 
shafts  coupled  directly  together.  One  of  the  dynamos,  ar- 
ranged as  a  motor,  is  driven  by  the  current  from  the  two 
outside  wires  of  the  three- wire  system  by  a  potential,  there- 
fore, of  about  216  volts.  The  circuit  is  run  to  the  trans- 
former from  the  central  switchboard.  The  dynamo,  which 
is  coupled  to  the  motor,  and  furnishes  the  low-potential 
current,  is  so  arranged  that  the  potential  at  the  poles  may 
be  varied  from  about  4.5  to  9  volts,  the  corresponding  cur- 
rent-strengths being  respectively  360  and  180  amperes.  The 
conductors  carrying  the  low-potential  current  from  the  dy- 
namo run  to  the  central  switchboard.  The  potential  of  9 
volts  is  the  one  generally  used,  the  lower  potential  of  4.5 
volts  being  employed  for  larger  electrolytic  experiments, 
such  as  the  preparation  of  pure  metals. 

The  alteration  in  the  potential  of  the  current  is  brought 
about  by  connecting  the  two  halves  of  the  double  arma- 
ture, with  which  the  dynamo  is  provided,  either  in  series 
or  in  parallel.  This  is  done  by  merely  changing  the  corre- 
sponding connections  on  the  frame  of  the  transformer. 

Further,  concerning  the  construction  of  the  transformer, 
it  should  be  mentioned  that  the  machine  is  very  solidly  built 
and  the  magnets  protected  within  the  frame,  so  that  a  me- 
chanical injury  to  the  field-coils  is  out  of  the  question.  The 
lubrication  of  all  parts  is  carried  out  by  means  of  ring-lubri- 
cation, which  has  proved  very  satisfactory.  Such  delays 
as  often  occur  when  other  mechanical  contrivances  are  em- 


I 

K 


ARRANGEMENTS    FOR    ANALYSIS.  147 

ployed  are  here  impossible.  Owing  to  its  construction,  the 
transformer,  which  for  protection  is  enclosed  in  a  special 
covering,  can  run  for  hours  without  particular  attention. 

The  action  of  the  transformer,  in  spite  of  its  speed  of 
about  1300  revolutions  per  minute,  is  so  quiet  and  free  from 
any  jarring  or  shaking,  that  its  running  can  scarcely  be  de- 
tected even  in  the  immediate  neighborhood. 

It  should  be  stated  that  there  is  a  switchboard  near  the 
transformer,  by  which  direct  currents  of  low  potential  can  be 
taken  off  in  this  room  without  making  use  of  the  central 
switchboard.  Such  currents  are  required  when  experiments 
with  high-current  strength  and  low  potential  are  performed; 
and  in  such  cases  short  cables  are  run  from  this  switchboard 
to  the  nearest  work-bench,  where  the  apparatus  is  set  up. 

The  arrangement  of  the  large  work-bench,  a  photograph 
of  which  is  given  in  Plate  IV,  corresponds  in  general  to  that 
of  the  table  for  conducting  analyses  in  the  private  laboratory. 

Here,  on  either  side  of  the  bench,  there  are  five  work- 
places, at  each  of  which  two  analyses  can  be  performed 
simultaneously,  so  that  in  all  twenty  experiments  can  be 
carried  on  at  the  same  time.* 

Plate  V  shows  the  method  employed  for  measuring 
the  current-strength  and  potential  of  an  analysis.  The  am- 
peremeter and  voltmeter  are  above.  The  currents  are  reg- 
ulated by  means  of  the  rheostats  (I,  II,  III,  and  IV).  These 
consist  of  slate  blocks  into  which  are  fixed  metal  knobs 
attached  to  separate  resistance  spirals.  By  turning  the 
lever  in  the  direction  indicated  by  the  arrow,  the  resistance 
is  cut  out  and  the  current-strength  correspondingly  increased. 

The  switches  for  the  amperemeter  A(i  u  m,  iv>,  for  the 


*  Two  other  work-benches  have  been  recently  added,  so  that  there  are 
now  twenty  work-places  for  electroanalysis. 


148  .QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

electrolyses  E(it  n,  in,  iv>,  and  the  safety-fuses  B(i  n  m,  iv> 
are  covered  by  bronzed  metal  cases. 

0(i,  n(  mt  iv)  are  the  binding-posts  to  which  the  elec- 
trolyses are  connected,  V  is  a  double-pole  switch  used  in 
measuring  the  potential.  In  the  position  o  the  voltmeter 
is  cut  out;  at  i,  n;  m,  iv  the  corresponding  electrolysis  is 
connected  with  the  voltmeter.  As  already  stated,  there  is 
only  one  amperemeter  and  one  voltmeter  to  every  table  with 
10-20  dishes,  and  therefore  only  one  electrolysis  can  be 
measured  at  'a  time.  The  four  figures  in  Plate  V  are  de- 
signed to  make  the  explanations  clearer. 

In  position  I,  where  the  keys  AI  and  EI  are  horizontal, 
the  circuit  is  open.  In  II,  An  is  vertical;  the  amperemeter 
is  connected.  If  the  key  E  is  now  turned  to  the  vertical 
position  a  current  will  flow  through  the  circuit  and  the  lever 
of  the  rheostat  at  II  may  be  turned  in  the  direction  of  the 
arrow  until  the  amperemeter  registers  the  desired  current- 
strength.  A  is  then  brought  into  the  position  Am.  The 
current  now  flows  no  longer  through  the  amperemeter,  but 
through  a  roll  of  wire,  the  resistance  of  which  is  equal  to  that 
of  the  amperemeter.  The  current-strength  remains  the 
same  as  that  previously  shown  by  the  amperemeter. 

V  serves  for  measuring  the  potential  at  the  electrodes 
of  the  electrolytic  vessel,  as  shown  at  Vrv-  In  this  operation 
the  position  of  A  and  E  is  the  same  as  in  III.  The  two  metal 
strips  (SS)  are  pushed  to  the  right  or  left  (in  the  figure  to 
the  right,  iv),  and  the  voltmeter  then  shows  the  potential 
existing  at  that  time  between  the  electrodes  of  the  corre- 
sponding electrolysis.  The  measuring  instruments  are 
switched  out  of  the  circuit  immediately  after  use. 


']=? 


f 


ARRANGEMENTS  FOR  ANALYSIS.  149 


4.  LABORATORY  FOR  PERFORMING  EXPERIMENTS  ON  A  LARGE 
SCALE  WITH  Low  AND  HIGH  POTENTIALS. 

As  already  mentioned,  special  cases  which  receive  their 
currents  from  separate  conductors  running  from  the  central 
switchboard  are  provided  for  high  and  low  potential. 

Within  the  case  for  high  potential  there  are  three  separate 
plates  corresponding  to  the  three  feed-wires  of  the  three-wire 
system,  providing  currents  at  potentials  of  108  and  216  volts 
accordingly. 

The  case  for  low  potential  contains  two  connections,  with 
possible  potential  at  the  poles  up  to  9  volts. 

From  both  of  the  cases  separate  branch  circuits  run  to 
the  four  work-benches,  where  they  end  in  terminal  boxes 
provided  with  locks.  By  this  arrangement  each  table  is 
provided  with  both  high  and  low  potential. 

Each  of  the  branches  running  to  the  tables  is  supplied 
with  a  safety-fuse  and  a  switch;  each  table  is  therefore 
independent  of  the  others. 

In  conducting  experiments  a  set  of  portable  measuring 
instruments  and  portable  resistances  for  regulating  the  cur- 
rent is  used. 

Large  and  cumbersome  resistances  are  required  to  pro- 
duce appreciable  variations  in  the  potential.  A  simple 
appliance  in  use  in  the  Aachen  laboratory  overcomes  this 
difficulty  in  the  case  of  experiments  of  short  duration,  where 
economical  use  of  the  current  is  not  an  essential  feature. 
This  scheme,  originated  by  Lob  and  Kaufmann,*  permits 
the  convenient  splitting  up  of  the  current  of  216  or  108  volts 
into  separate  independent  currents  having  the  required 
lower  potential. 

*  Zeit.  f.  Elektrochem.,  2,  345  (1895-96);  ibid.,  2,  664. 


150 


QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


FIG.  91. 


A  number  of  lead  plates  are  hung  parallel  to  one  another 
in  a  large  porcelain  trough  filled  with  sulphuric  acid  (Fig.  91), 

in  such  a  manner  that  they  cut  all 
the  lines  of  the  current.  They 
must  therefore  almost  touch  the 
sides  and  bottom  of  the  trough. 
When  the  current  passes,  these 
lead  plates  act  as  intermediate 
conductors,  the  sum  of  their 
separate  potentials  being  equal  to 
the  potential  of  the  main  current. 
The  immersed  lead  plates  can  be  slid  along  the  length  of 
the  trough  on  the  glass  rod  from  which  they  are  hung.  By 
moving  the  plates  toward  or  away  from  the  electrodes  the 
potential  is  varied,  and  any  desired  potential  may  be  ob- 
tained by  making  a  connection  between  a  terminal  electrode 
and  one  of  the  plates.  The  arrangement  is  given  in  Fig.  91. 
E  denotes  the  source  of  current;  T,  the  trough  filled  with 
sulphuric  acid;  A  and  K,  anode  and  cathode;  M,  the  five 
plates.  The  wires  to  S  show  the  removal  of  three  separate 
currents  of  different  potentials.  A  large  number  of  such 
connections  are  possible.  On  account  of  the  gases  given 
off,  the  trough  should  be  kept  under  a  hood. 

In  addition  to  the  details  of  the  equipment  which  have 
been  described,  some  general  facts  in  connection  with  the 
management  of  the  entire  plant  should  be  stated. 

Since  the  apparatus  is  much  used,  and  is  not  always 
placed  in  experienced  hands,  it  was  considered  desirable  to 
have  all  parts  solidly  constructed  and  intended  for  contin- 
uous use. 

The  switches  and  regulating  instruments,  as  well  as  the 
branch-plates,  are  all  mounted  on  bases  of  fire-proof 
material. 


,}>=== 


0  f 
2  >, 

1  ^ 
a  H 
a 

I? 


ARRAXGEMEXTS    FOR    AXALYSIS.  151 

All  connections  are  made  with  the  best  rubber-covered 
wire,  fastened  to  large  porcelain  brackets,  so  that  most  perfect 
insulation  of  the  conductors  is  assured. 

To  secure  against  improper  use,  all  switch-cases  are  pro- 
vided with  safety-locks,  so  that  currents  can  nowhere  be 
taken  off  without  the  permission  of  the  director  of  the 
laboratory. 


PART  SECOND. 

SPECIAL. 

SECTION  I. 

QUANTITATIVE  DETERMINATION.  OF  METALS. 


IRON. 

LITERATURE : 

Wrightson,  Zeit.  f.  analyt.  Chem.,  15,  305  (1876). 
Parodi  and  Mascazzini,  Zeit.  f.  analyt.  Chem.,  18,  588  (1879). 
Luckow,  Zeit.  f.  analyt.  Chem.,  19,  18  (1879). 
Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  (1881). 
Moore,  Chem.  News,  53,  209  (1886). 
Smith,  Amer.  Chem.  Jour.,  10,  330  (1888). 
Brand,  Zeit.  f.  analyt.  Chem.,  28,  581  (1889). 

Drown  and  Meckenna,  J.  of  Analyt.  and  Applied  Chem.,  5,  627  (1891). 
Smith  and  Muhr,  ibid.,  5,  488  (1891). 
Rudorff,  Zeit.  f.  angew.  Chem.,  15,  198  (1892). 
Vortmann,  Monatshefte  f.  Chem.,  14,  542  (1893). 
Classen,  Zeit.  f.  Elektrochemie,  i,  229  (1894). 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  29,  1585  (1896). 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 
Avery  and  Dales,  Ber.  deutsch.  chem.  Ges.,  32,  64  and  2233  (1899). 
Verwer  and  Goll,  ibid.,  32,  806  (1899). 

Verwer,  Chemiker-Zeitung,  25,  No.  75  (1901);  Jahrbuch  Elektro- 
chem.,  6,  246. 

If  a  solution  of  a  ferrous  salt  is  treated  with  potassium  or 
ammonium  oxalate,  there  is  produced  an  intensely  yellowish- 
red  precipitate  of  ferrous  oxalate,  soluble  in  an  excess  of  the 
reagent  to  a  yellowish-red  solution  of  the  double  salt. 

153 


154          QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

The  stated  oxalates  do  not  precipitate  ferric  salts;  but, 
if  added  in  sufficient  quantity,  a  solution  of  the  double  ferric 
salt  is  produced  having  a  more  or  less  green  color.  If  this 
solution  is  submitted  to  electrolysis,  there  is  first  produced 
the  double  ferrous  salt,  which  is  then  decomposed  with  separa- 
tion of  metallic  iron ;  the  green  liquid  therefore  becomes  first 
red  and  then  colorless.  Because  of  this  action,  the  deter- 
mination of  iron  is  more  rapidly  performed  in  solutions  of 
ferrous  than  of  ferric  salts.  Potassium  iron  oxalate  is  not 
adapted  to  electrolysis,  because  the  potassium  carbonate 
which  is  produced  precipitates  iron  carbonate,  and  thus  com- 
plete reduction  is  prevented.  The  electrolysis  of  the  ammo- 
nium double  -  salt,  when  ammonium  oxalate  is  in  sufficient 
excess,  proceeds  smoothly,  with  no  separation  of  an  iron 
compound.  If  the  solution  contains  free  hydrochloric  acid, 
it  is  best  to  remove  it  by  evaporation  on  the  water- 
bath. 

Free  sulphuric  acid  may  be  neutralised  with  ammonia, 
since  the  ammonium  sulphate  thus  produced  only  increases 
the  conductivity  of  the  solution.  Nitrates  are  converted  by 
evaporation  with  sulphuric  acid  into  sulphates,  or  by  repeated 
evaporation  with  hydrochloric  acid  into  chlorides.  The 
presence  of  phosphoric  acid  is  not  objectionable. 

The  determination  is  conducted  as  follows:  Assuming 
that  1  g  of  iron  may  be  present  in  the  solution  to  be  elec- 
trolysed, from  6  to  8  g  of  ammonium  oxalate  are  dissolved 
by  heating  in  as  little  water  as  possible,  and  with  constant 
stirring  the  iron  solution  is  gradually  added.*  The  solution 
is  then  diluted  with  water  to  100-150  cc  and  the  positive 

*  It  is  not  desirable  to  add  ammonium  oxalate  solution  to  a  ferrous 
solution,  as  difficultly  soluble  ferrous  oxalate  separates,  and  can  be  dis- 
solved to  the  double  salt  only  by  long  heating.  With  a  ferric  solution  this 
precaution  is  unnecessary. 


IRON.  155 

electrode  is  immersed  in  the -liquid  until  it  is  just  covered 
by  the  solution.  The  electrolysis  is  conducted  according 
to  the  special  directions  which  are  given  below. 


CONDITIONS   FOR    ANALYSIS. 

Metal  present  as  sulphate. 
Substance  added:  6  to  8  g  ammonium  oxalate. 
Total  volume  of  solution:  100  to  150  cc. 
Temperature:  that  of  room,  or  40°  to  65°. 
Current-density  at  cathode: 

(Room  temp.)  ND100  =  1.0  to  1.5  amp.; 
(40°  to  65°)  ND100  =  0.5  to  1.0  amp. 

Potential-difference  between  electrodes: 
(Cold  solutions)  3.6  to  4.3  volts; 
(Warm  solutions)  2.0  to  3.5  volts. 

Time  required:  2\  to  6J  hours,  depending  on  the  tem- 
perature. For  the  quality  of  the  precipitated  metal,  polished 
or  roughened  dishes  answer  equally  well. 

The  end  of  the  reaction  is  determined  by  taking  out  a 
small  portion  of  the  colorless  solution  with  a  capillary  tube, 
acidifying  strongly  with  hydrochloric  acid,  and  testing  with 
potassium  sulphocyanide.  When  the  reaction  is  ended  the 
positive  electrode  is  removed  from  the  solution,  which  is 
poured  off,  and  the  dish  washed  three  times  with  cold  water 
(about  5  cc  each  time),  and  three  times  with  absolute  alcohol, 
dried  a  few  moments  in  the  air-bath  at  a  temperature  of  70° 
to  90°,  and  weighed  after  cooling. 

The  separated  iron  has  a  steel-gray  color  and  brilliant 
luster,  is  firmly  attached  to  the  dish,  and  can  be  preserved  in 
the  air  without  oxidation  for  a  full  day. 


156  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


EXPERIMENT    1. 

Used    2.1-2.5   g    FeS04(NH4)2S04.6H20  (Fe  =  14.29%), 
6-8  g  ammonium  oxalate,  120  cc  of  liquid. 

CUF  Amptref  ^  Electr°vdoelt^tentia1'  Temp.                    Time.  Found. 

1     -1.5               3.85-4.3  20-40°  2  hr.  15  m.  14.21% 

1     -1.05             3.6-4.2  36°  3   "    50"  14.21    " 

1     -1.08             3.05-3.52  65°  2   "    30  "  14.28    " 

0.5-0.55             2.0-2.3  50-52°  3    M    30"  14.24    " 

EXPERIMENT    2. 

Used    2.6-2.8    g    ferric    potassium    oxalate   (Fe2(C204)3. 
3K2C204.6H20)  (Fe  =  11.40%),  6-7  g  ammonium  oxalate. 


Current-density, 
Amperes. 

1.5-1.7 
1.0-1.1 

Electrode  Potential, 
Volts. 

3.55-4.25 
3.9  -4.0 

Temp. 
35-40° 
30-40° 

Time. 
2  hr.  54  m. 

3    "    15  " 

Found. 
11.39  % 
11.35    " 

0.5-0.8 

2.4  -2.8 

50° 

6    "    15  " 

11.25    " 

Edgar  F.  Smith  has  recommended  the  precipitation  of 
iron  from  a  solution  of  ammonium  citrate  to  which  a  few 
drops  of  citric  acid  have  been  added.  The  author's  experi- 
ments in  earlier  years,  on  the  separation  of  iron  from  other 
metals  in  citric  and  tartaric  acid  solution,  demonstrated 
that  in  the  presence  of  fixed  organic  acids  the  precipitated 
metal  always  contains  carbon.  Heidenreich  has  shown,  by 
experiments  conducted  in  the  Aachen  laboratory,  that  iron 
may  be  quantitatively  determined  from  such  solutions  under 
certain  conditions,  namely:  0.2  g  ferrous  ammonium  sul- 
phate, 50  cc  of  a  10  per  cent,  solution  of  sodium  citrate.  2  cc 
of  a  saturated  solution  of  citric  acid;  entire  volume  of  liquid, 
120  cc;  temperature  of  room ;  ND100  =  0.75-0. 9  amp.;  poten- 
tial-difference, 5  volts;  time,  4-6  hours.  The  iron,  however, 
always  contains  carbon. 


COBALT.  157 


COBALT. 

LITERATURE: 

Luckow,  Dingl.  Polyt.  Journ.,  177,  235  (1850). 

Gibbs,  Zeit.  f.  anal.  Chem.,  3,  336  (1864);  n,  10(1872);  22,  558  (1883). 

Merrick,  Amer.  Chemist,  2,  136  (1871). 

Mansfeld.  O.  Berg.  Hiittendirektion,  Zeit,  f.  anal.  Chem.,  n,  1  (1872). 

Hampe,  Zeit.  f.  anal.  Chem.,  13,  186  (1874). 

Herpin,  Zeit.  f.  anal.  Chem.,  15,  535  (1876). 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  300,  303,  333  (1876). 

Schweder,  ibid.,*i6,  344  (1877). 

Cheney  and  Richards,  Amer.  Journ.  of  Science,  [3]  14,  178  (1877) 

Ohl,  Zeit.  f.  anal.  Chem.,  18,  523  (1879). 

Fresenius  and  Bergmann,  Zeit.  f.  anal.  Chem.,  19,  329  (1880). 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  314  (1880). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622,  2771  (1881). 

Riche,  Zeit.  f.  anal.  Chem.,  21,  116  (1882). 

Schucht,  Zeit.  f.  anal.  Chem.,  21,  493  (1882). 

Moore,  Chem.  News,  52,  209  (1885). 

Kohn  and  Woodgate,  Journ.  Soc.  Chem.  Indust,,  8,  256  (1889). 

Brand,  Zeit.<f.  anal.  Chemie,  28,  588  (1889). 

Le  Roy,  Compt.  rend.,  112,  722  (1891). 

Riidorff,  Zeit.  f.  angew.  Chemie,  1892,  p.  6. 

Smith  and  Muhr,  Journ.  Anal.  Chem.,  5,  488  (1891) ;  ibid.,  7,  189  (1893). 

Vortmann,  Monatsh.  f.  Chem.,  14,  536  (1893). 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2061  (1894). 

Oettel,  Zeit.  f.  Elektrochemie,  i,  192  (1894-95). 

Winkler,  Zeit.  f.  anorg.  Chem.,  8,  291  (1895). 

Campbell  and  Andrews,  Journ.  Am.  Chem.  Soc.,  17,  125  (1895). 

Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 

Foster,  Zeit.  f.  Elektrochem.,  6,  160  (1899-1900). 

Cobalt  may  be  very  easily  precipitated  from  a  solution 
of  cobalt  ammonium  oxalate  containing  an  excess  of  ammo- 
nium oxalate  (method  of  the  author).  The  metal  separates 
rapidly  at  the  negative  electrode  in  a  compact  adherent 
coating,  showing  its  characteristic  metallic  properties.  The 
operation  is  performed  as  in  the  determination  of  iron.  4-5 
g  ammonium  oxalate  are  dissolved  by  heating  in  the  solution, 


158  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

the  volume  of  which  should  be  about  25  cc ;  it  is  then  diluted 
to  100-120  cc,  warmed,  and  electrolysed  at  60-70°. 

CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  sulphate. 

Substance  added:  4  to  5  g  ammonium  oxalate. 

Total  volume  of  solution:   100  to  120  cc. 

Temperature:  60°  to  70°. 

Current-density  at  cathode:  ND100  =  1.0  ampere. 

Potential-difference:  3.1  to  3.8  volts. 

Time  required :  2 J  to  3J  hours. 

The  state  of  the  surface  of  the  cathode  (whether  rough  or 
smooth)  has  no  influence  on  the  quality  of  the  precipitated 
metal. 

EXPERIMENT. 

Used  2.2-2.6  g  CoS04.K2S04.6H2O  (Fe  =  13.43%),  4-5  g 
ammonium  oxalate,  120  cc  solution. 


rrent-clensity,    ± 
Amperes. 

jlectroae  r'otenti! 
Volts. 

*•       Temp. 

Time. 

•  Found. 

1     -1.1 

3.1  -3.78 

60-65° 

2  hr.  15  m. 

13.36  % 

0.5-0.52 

2.7  -2.95 

60-65° 

3    "    30  " 

13.49  " 

1    -1.2 

3.9  -4.0 

15-35° 

4   "    30  " 

13.43  " 

0.5-0.53 

3.46-3.9 

15-27° 

6   "    35" 

13.25  " 

According  to  a  method  given  by  Fresenius  and  Bergmann, 
the  cobalt  solution,  after  the  addition  of  15-20  cc  of  an  am- 
monium sulphate  solution  (300  g  (NH4)2S04  to  the  liter) 
and  40  cc  ammonia  sp.  gr.  0.96  (where  more  than  0.5  g  cobalt 
is  present  in  the  solution,  50-60  cc  NH4OH)  is  diluted  with 
water  to  150-170  cc,  and  electrolysed  with  a  current  of  ND100 
=  0.7  as  a  maximum  at  ordinary  temperatures.  The  presence 
of  chlorides  and  nitrates  is  unfavorable  to  the  reduction. 
Fixed  organic  acids  (citric  acid,  tartaric  acid)  and  also  mag- 


XICKEL.  159 

nesiurn  compounds  act  injuriously.     The  presence  of  phos- 
phates is  not  objectionable. 


CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  sulphate. 

Substance  added:  4.5  to  6  g  ammonium  sulphate  and 
40  to  60  cc  ammonia  (sp.  gr.  =0.96). 

Total  volume  of  solution:  150  to  170  cc. 

Temperature :  that  of  room. 

Current-density:  ND100  =  0.5  to  0.7  ampere. 

Potential-difference:  2.8  to  3.3  volts. 

Time  required:  about  six  hours. 

F.  Oettel  has  proposed  the  following  method  for  the 
determination  of  cobalt: 

The  salt  is  dissolved  in  water  and  a  quantity  of  ammo- 
nium chloride,  equal  to  four  times  the  weight  of  the  salt  taken, 
is  added.  The  final  volume  of  the  liquid  should  be  150  cc, 
\  of  which  is  an  ammonia  solution  (sp.  gr.  =  0.92).  (See 
further  under  Nickel.) 

NICKEL. 

LITERATURE  I 

Luckow,  Dingl.  Polyt.  Journ.,  177,  235  (1850). 

Gibbs,  Zeit.  f.  anal.  Chem.,  3,  336  (1864) ;  n,  10  (1872) ;  22,  558  (1883). 

Merrick,  Amer.  Chemist,  2,  136  (1871). 

Mansfeld.  O.  Berg.  Huttendirektion,  Zeit.  f.  anal.  Chem.,  u,  1  (1872) 

Hampe,  Zeit.  f.  anal.  Chem.,  13,  186  (1874). 

Herpin,  Zeit.  f.  anal.  Chem.,  15,  535  (1876). 

AVrightson,  Zeit.  f.  anal.  Chem.,  15,  300,  303,  333  (1876). 

Schweder,  ibid.,  16,  344  (1877). 

Cheney  and  Richards,  Amer.  Journ.  of  Science,  [3]  14,  178  (1877). 

Ohl,  Zeit.  f.  anal.  Chem.,  18,  523  (1879). 

Fresenius  and  Bergmann,  Zeit.  f.  anal.  Chem.,  19,  329  (1880). 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  314  (1880). 


160          QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622,  2771  (1881). 

Rich6,  Zeit.  f.  anal.  Chem.,  21,  116  (1882). 

Schucht,  Zeit.  f.  anal.  Chem.,  21,  493  (1882). 

Moore,  Chem.  News,  52,  209  (1885). 

Kohn  and  Woodgate,  Journ.  Soc.  Chem.  Indust.,  8,  256  (1889). 

Brand,  Zeit.  f.  anal.  Chemie,  28,  588  (1889). 

Le  Roy,  Compt.  rend.,  112,  722  (1891). 

Riidorff,  Zeit.  f.  angew.  Chemie,  1892,  p.  6. 

Smith  and  Muhr,  Journ.  Anal.  Chem.,  5,  488  (1891) ;  ibid.,  7,  189  (1893). 

Vortmann,  Monatsh.  f.  Chem.,  14,  536  (1893). 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2061  (1894). 

Oettel,  Zeit.  f.  Elektrochemie,  i,  192  (1894-95). 

Winkler,  Zeit.  f.  anorg.  Chem.-,  8,  291  (1895). 

Campbell  and  Andrews,  Journ.  Am.  Chem.  Soc.,  17,  125  (1895). 

Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 

Foster,  Zeit.  f.  Elektrochem.,  6,  160  (1889-1900). 

Gooch  and  Medway,  Am.  Journ.  of  Science,  April,  1903. 

Nickel  may  be  reduced  under  .conditions  similar  to  those 
for  the  determination  of  cobalt;  the  metal  is  precipitated 
from  a  solution  of  double  oxalates,  containing  ammonium 
oxalate  in  excess,  as  a  bright  adherent  coating  on  the  negative 
electrode. 

CONDITIONS   FOR    ANALYSIS. 

Metal  present  as  sulphate. 

Substance  added :  6  to  8  g  ammonium  oxalate. 

Total  volume  of  solution:  120  cc. 

Temperature:  60°  to  70°. 

Current-density  at  cathode:   ND100  =  1.0  ampere. 

Potential-difference :  3  to  4  volts. 

Time  required:  about  3  hours. 

The  question  of  the  complete  precipitation  of  the  nickel 
can  be  settled  by  adding  a  small  quantity  of  ammonium 
sulphide  to  the  solution  and  concentrating  the  solution  by 
evaporation,  when  any  nickel  remaining  will  be  precipitated 
as  sulphide.  If  the  quantity  of  this  is  appreciable  it  can 
be  determined  by  adding  bromine  to  dissolve  the  sulphide 


NICKEL.  161 

and  again  electrolysing  the  solution  after  the  addition  of 
an  excess  of  ammonia. 

According  to  Fresenius  and  Bergmann,  nickel  can  be 
completely  precipitated  from  a  solution  containing  ammo- 
nium sulphate  and  free  ammonia  (see  Cobalt). 

Oettel  has  demonstrated  that  nickel  may  also  be  deter- 
mined in  solutions  containing  the  chloride.  Since  the  pres- 
ence of  nitric  acid  is  very  objectionable,  however,  this  must 
be  entirely  removed  if  present.  Oettel  effected  this  by  evap- 
orating the  nitric  acid  solution  to  complete  dryness  and  then 
boiling  the  residue  several  times  with  concentrated  hydro- 
chloric acid  in  a  long-necked  flask,  until  the  reaction  of  nitric 
acid  with  diphenylamine  was  no  longer  obtained.  Small 
quantities  of  nickel  nitrate  can  be  precipitated  as  hydroxide, 
washed  and  dissolved  in  hydrochloric  acid.  Large  quantities 
are  most  conveniently  converted  into  sulphate  and  deter- 
mined by  one  of  the  methods  mentioned  above. 

To  the  solution  of  the  chloride  so  much  ammonia  (sp.  gr. 
=0.92)  is  added  that  10%  by  volume  of  free  ammonia  is 
present  (when  less  is  present  black  oxide  of  nickel  separates 
on  the  anode).  About  10  g  ammonium  chloride  per  gram 
of  metal  to  be  precipitated  is  added,  and  the  electrolysis  is 
conducted  with  currents  of  ND100  =  0.4  ampere.  The  time 
required  is  from  7  to  8  hours,  and  the  method  permits  large 
quantities  of  nickel  (as  much  as  2  grams)  to  be  precipitated 
in  white,  strongly  adherent  deposits.  A  uniform  current- 
density  on  all  portions  of  the  surface  of  the  cathode  is  im- 
portant to  the  success  of  this  method.* 

E.  F.  Smith  f  recommends  the  precipitation  of  nickel 
and  cobalt  from  a  solution  containing  an  alkali  cyanide 


*  Classen,  Ausgewahlte  Methoden,  p.  410. 
f  Electro-Chemical  Analysis,  1902,  p.  94. 


162  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

under  the  following  conditions:  Enough  potassium  cyanide 
is  added  to  the  solution  of  the  metal  salt  to  redissolve  the 
precipitate  at  first  formed  and  to  provide  an  excess  of  0.1  gram 
of  the  cyanide;  2  grams  of  ammonium  carbonate  are  then 
added,  the  solution  is  diluted  to  150  cc,  warmed  to  60°,  and 
electrolysed  with  a  current  of  ND100  =  1.5  ampere  and  a  po- 
tential-difference of  6  to  6.5  volts.  The  time  required  for 
complete  precipitation  is  about  three  and  one-half  hours. 

According  to  Fernberger  and  Smith  the  determination 
of  nickel  may  also  be  conducted  in  a  solution  containing 
phosphates.  They  mention  the  following  experiment: 

To  a  solution  of  the  nickel  salt  (containing  0. 1360  g  of  Ni) 
45  cc  of  a  disodium  hydrogen  phosphate  solution  (sp.  gr.  = 
1.0358)  and  enough  phosphoric  acid  to  dissolve  the  precipi- 
tate formed  and  to  have  a  few  drops  in  excess  were  added. 
The  solution  was  then  diluted  to  250  cc,  warmed  to  65°,  and 
electrolysed  for  three  and  one-half  hours  with  a  current 
of  ND100  =  0.53  ampere  and  a  potential-difference  of  7 
volts. 

Campbell  and  Andrews  dissolve  nickel  hydroxide  in  30  cc 
of  a  10  per  cent,  solution  of  disodium  hydrogen  phosphate,  to 
which  30  cc  of  a  concentrated  ammonia  solution  are  added, 
and,  with  a  distance  of  5  mm  between  the  electrodes,  separate 
the  nickel  by  the  use  of  a  current  of  ND100  =  0.14  amp. 

Gooch  and  Medway  have  used  the  apparatus  described 
on  p.  124  for  the  determination  of  nickel.  Nickel  ammo- 
nium sulphate  was  dissolved  in  25  cc  of  water  and  20  cc  of 
strong  ammonia  were  added.  In  this  solution  about  1  g  of 
ammonium  sulphate  was  dissolved  and  the  electrolysis  was 
conducted  with  currents  of  from  1.5  to  4  amperes  (equivalent 
to  ND100  =  5  — 13.3  amperes).  The  time  required  for  the 
complete  precipitation  of  the  nickel  (0.0954— 0.1738  g)  wras 
from  20  to  30  minutes. 


ZINC.  163 

ZINC. 

LITERATURE : 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  303  (1876). 

Parodi  and  Mascazzini,  Ber.  deutsch,  chem.  Ges.,  10,  1098  (1877); 

Zeit.  f.  anal.  Chem.,  18,  588  (1879). 
Riche",  Zeit.  f.  anal.  Chem.,  17,  216  (1878). 
Beilstein  and  Jawein,  Ber.  deutsch,  chem.  Ges.,  12,  446  (1879); 

Zeit.  f.  anal.  Chem.,  18,  588  (1879). 

Reinhardt  and  Ihle,  Journ.  f.  prakt.  Chem.,  24,  193  (1881). 
Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  (1881). 
Riche,  Zeit.  f.  anal.  Chem.,  21,  119  (1882). 
Millot,  Bull,  de  la  Soc.  chim.,  37,  339  (1882). 
Gibbs,  Zeit.  f.  anal.  Chem.,  22,  558  (1883). 
Moore,  Chem.  News,  52,  209  (1885). 
Luckow,  Zeit.  f.  anal.  Chem.,  25,  113  (1886). 
v.  Malapert,  Zeit.  f.  anal.  Chem.,  26,  56  (1887). 
Herrick,  Journ.  Anal.  Chem.,  2,  167  (1888). 
Brand,  Zeit.  f.  anal.  Chem.,  28,  581  (1889). 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2753  (1891)." 
Warwick,  Zeit.  f.  anorg.  Chem.,  i,  290  (1892). 
Rudorff,  Zeit.  f.  angew.  Chem.,  1892,  p.  197. 
Vortmann,  Monatsh.  f.  Chemie,  14,  536  (1893). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Jordis,  Zeit.  f.  Elektrochemie,  2, 138, 565, 655  (1895-96). 
Nicholson  and  Avery,  Journ.  Am.  Chem.  Soc.,  18,  659  (1896). 
Paweck,  Zeit.  f.  Berg-u.  Hiittenwesen,  46,  570  (1898). 
Riderer,  Journ.  Am.  Chem.  Soc.,  21,  789  (1899)! 
Smith,  Journ.  Am.  Chem.  Soc.,  24,  1073  (1902). 

The  metal  may  be  easily  and  quickly  separated  from  the 
double  salts  of  zinc  ammonium  oxalate  and  zinc  potassium 
oxalate  (method  of  the  author).* 

The  reduced  metal  has  a  bluish-white  color,  and  under 
proper  conditions  adheres  firmly  to  the  negative  electrode. 

*  The  reduction  of  zinc  from  a  solution  of  zinc  ammonium  oxalate  is  very 
often  credited  to  Reinhardt  and  Ihle.  The  author,  however,  described 
this  method  in  Fehling's  "  Handworterbuch "  before  the  publication  of  the 
article  by  Reinhardt  and  Ihle  in  the  Journal  fur  praktische  Chemie,  to 
the  editor  of  which,  Kolbe,  the  author  especially  stated  the  facts  at  the  time. 


164  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Indeed,  the  metallic  zinc  often  adheres  so  firmly  to  the  plat- 
inum dish  that,  after  being  cleaned  with  water  and  alcohol 
and  dried,  it  is  with  difficulty  dissolved  by  warming  with 
acids.  Generally,  after  this  operation,  a  dark  coating  of 
platinum-black  remains  which  can  only  be  removed  by  ignit- 
ing the  dish  and  again  treating  with  acids.  It  is  therefore 
desirable,  before  weighing  the  dish,  to  precipitate  upon  it  a 
thin  coating  of  copper,  tin,  or,  better,  silver.  In  laboratories 
where  many  zinc  determinations  are  performed,  silver  dishes 
may  be  advantageously  employed. 

A  bright,  thick  coating  of  copper  can  be  obtained  in  a  few 
minutes  if  a  saturated  solution  of  copper  sulphate  is  treated 
with  an  excess  of  ammonium  oxalate  to  form  the  double  salt, 
acidified  with  oxalic  acid,  warmed  to  70-80°,  and  the  copper 
precipitated  by  a  current  of  1  ampere.  The  preparation  of 
the  double  salt  in  a  beaker,  and  the  transfer  of  the  clear 
hot  solution  to  the  platinum  dish  is  to  be  recommended. 

For  silvering  the  dish  it  is  best  to  precipitate  the  silver 
from  a  solution  containing  potassium  cyanide  (see  Silver). 

In  determining  zinc  by  this  method,  the  zinc  salt  is  dis- 
solved in  a  little  water  by  warming,  about  4  g  of  potassium 
oxalate  or  an  equal  amount  of  ammonium  oxalate  is  added 
and  the  whole  is  brought  into  solution  by  warming  and,  if 
necessary,  by  the  addition  of  small  quantities  of  water.* 
The  liquid  is  now  transferred  to  a  platinum  dish  coated  with 
copper  or  silver  and  electrolysed.  The  author  has  demon- 
strated by  experiments  that  the  separation  of  the  zinc  in  a 
dense,  metallic  form  is  possible  if  the  solution  be  kept  acid 
during  the  process  of  analysis. 

*  If  the  alkali  oxalate  be  added  to  a  dilute  solution  of  a  zinc  salt,  there 
first  forms  a  precipitate  of  zinc  oxalate  which  is  not  completely  converted 
into  the  soluble  zinc  double  salt  if  the  solution  of  the  alkali  oxalate  is  too 
dilute. 


ZINC.  165 

For  acidifying  the  solution,  a  cold  saturated  solution 
of  oxalic  acid,  or,  better,  a  solution  of  tartan  c  acid  (3  : 50) 
is  employed.  At  the  start  the  solution  is  electrolysed 
for  about  3-5  minutes  without  addition  of  acid,  and 
then  the  acid  is  permitted  to  flow  in  drops  (about  10 
drops  per  minute)  from  a  burette  with  a  fine  outlet,  upon  the 
watch-glass  covering  the  dish.  The  acid  flows  through  the 
holes  in  the  watch-glass  into  the  dish  itself.  After  the  re- 
duction is  completed  (this  is  determined  with  potassium 
ferrocyanide),  the  metal  must  be  washed  without  interrupt- 
ing the  current. 

CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  sulphate. 

Substance  added :  4  g  potassium  or  ammonium  oxalate. 

Total  volume  of  solution :  120  cc. 

Temperature:  50°  to  60°. 

Current-density  at  cathode : 

ND100=0.5  to  0.1  ampere. 
Potential-difference :  3.5  to  4.8  volts. 
Time  required :  about  2  hours. 

Roughened  or  polished  dishes  answer  equally  well,  but 
they  should  be  copper  or  silver  plated  before  use. 

EXPERIMENT. 

Used  1.8-2  g  zinc  ammonium  sulphate  (Zn  =  19.29%), 
4  g  potassium  oxalate,  120  cc  solution. 

CUrAmperesShy'    H^$j**entia1'        Temp.  Time.  Found. 

0.5-0.55  3.5-4.0  55-60°         2  hr.  16.44% 

0.9-1  4.7-4.8  60°  1    "    50m.         16.42    " 

According  to  v.  Miller  and  Kiliani,  4  g  potassium  oxalate 
and  3  g  potassium  sulphate  are  dissolved  in  water,  the  neu- 


166  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

tralised  zinc  solution  (sulphate  or  nitrate  containing  not 
more  than  0.3  g  Zn)  carefully  added,  and  electrolysis  effected 
without  heat,  by  a  current  of  ND100  =  0.3-0.5  ampere.  The 
reaction  is  complete  in  2  to  3  hours. 

N.  Eisenberg  *  obtained  the  following  results  by  the  pre- 
ceding method : 

«,,u  *  Current-        Electrode  Condition 

»UDst.  density,          Potential,        Temp.        Time.         Found.  of 

Taken.          Amperes.  Volts.  Metal. 

1.8312     0.4-0.35     3.9-4.0      25-26°      4  hr.      16.35%      partly  spongy 
1.8312     0.40-0.35     4.1-4.2      28-30°      4"       16.01"          spongy 
Remark:  (1)  Roughened  dish;    (2)  Polished  dish. 

The  constant  mixing  of  the  liquid  by  means  of  a  stirring 
appliance  is  recommended  for  this  method. 

According  to  Jordis,  zinc,  when  present  in  the  form  of 
sulphate,  chloride,  or  nitrate,  may  be  separated  from  a  lactic 
acid  solution.  The  ease  with  which  this  method  can  be  car- 
ried out  appears  from  the  directions  of  the  author,  which 
read  as  follows:  "2  g  ammonium  sulphate  and  5-7  g  ammo- 
nium lactate  are  added  to  the  neutral  solution  containing  riot 
less  than  0.3-0:5  g  zinc,  which  is  then  acidified  with  a  few 
drops  of  lactic  acid.  A  stirring  attachment  is  employed, 
and  the  solution  is  electrolysed  with  a  current  of  ND100=- 
1.0— 1.5  amp.  After  40-60  minutes  the  electrolyte  is  poured 
into  a  second  dish  and  the  separation  completed  in  this.  With 
a  current  of  the  above  density  this  requires  20-25  minutes. 
A  somewhat  concentrated  solution  of  about  120-150  cc  is 
advantageous." 

"  Since  the  lactic  acid  is  but  very  slowly  decomposed 
during  the  electrolysis,  its  regeneration  resulting  from  the 
action  of  the  sulphuric  acid  formed  upon  the  ammonium 
lactate,  the  electrolyte  remains  acid  until  the  end  and  re- 
quires no  further  attention. ' ' 

*  Inaugural-Dissert.  Heidelberg,  1895. 


ZINC.  167 

Luckow,  Beilstein  and  Jawein,  and  Millot  have  described 
the  precipitation  of  zinc  from  solutions  containing  potassium 
cyanide.  To  a  neutral  solution  of  the  zinc  salt,  or  a  solu- 
tion made  slightly  alkaline  with  sodium  hydroxide,  just 
enough  of  a  solution  of  pure  potassium  cyanide  is  added 
in  small  portions  to  dissolve  the  precipitate  of  zinc  cyanide 
at  first  formed,  and  the  solution  is  diluted  to  150  cc.  The 
current-density  can  be  from  0.5  to  0.1  ampere,  and  the  elec- 
trolysis may  be  conducted  at  ordinary  temperatures,  or  from 
50°  to  60°.  In  the  latter  case  the  difference  of  potential 
between  the  electrodes  will  be  from  5  to  8  volts  and  the  time 
required  from  two  to  two  and  one-half  hours.  If  the  deter- 
mination is  to  be  carried  out  over  night  weaker  currents 
can  be  used  at  room  temperature  and  a  good  deposit  will 
be  obtained.  The  completion  of  the  precipitation  can  be 
determined  by  decomposing  a  small  quantity  of  the  solution 
with  hydrochloric  acid  and  adding  potassium  ferrocyanide. 

A  method  previously  suggested  by  Vortmann  depended 
on  the  addition  of  a  known  weight  of  a  mercury  salt  to  the 
zinc  solution,  and  the  precipitation  of  the  mercury  and  zinc 
by  electrolysis  in  the  form  of  an  amalgam.  Paweck  has 
substituted  a  direct  method  for  this,  and  precipitates  the 
zinc  on  an  amalgamated  cathode. 

For  this  purpose  two  circular  disks  6  cm  in  diameter 
are  cut  from  ordinary  brass-wire  gauze,  and  after  thorough 
cleansing  by  scouring  and  by  treatment  with  acids  are  at- 
tached to  a  brass  wire  10  cm  long  and  1  mm  thick,  pointed 
at  the  lower  end,  in  such  a  manner  that  the  wire  extends 
about  2  mm  beyond  the  lower  disk,  and  the  two  disks  are 
.parallel  and  about  12  mm  apart.  After  washing  with  water, 
alcohol,  and  ether,  and  drying,  this  electrode  is  immersed 
in  a  solution  of  0.6  g  mercuric  chloride,  5  cc  concentrated 
nitric  acid,  200  cc  water,  and  electrolysed  for  f  to  1  hour 


168          QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

with  a  current  of  0.1  to  0.2  ampere.  By  this  treatment  the 
electrode  is  evenly  amalgamated,  and  it  is  then  washed  with 
hydrochloric  acid,  water,  alcohol,  and  ether,  and  dried  at  a 
gentle  heat,  as  is  obtained  by  holding  it  a  short  distance  from 
a  hot  asbestos  plate.  It  is  then  placed  on  a  watch-glass  in  a 
desiccator,  and  accurately  weighed  before  using  for  an  analysis. 

The  solution,  which  may  contain  as  much  as  0.5  g  of 
zinc  as  sulphate,  is  prepared  by  adding  10  g  of  sodium- 
potassium  tartrate  and  7  to  8  g  of  pure  sodium  or  potassium 
hydroxide  dissolved  in  water,  and  is  diluted  to  a  final  volume 
of  200  cc.  The  electrolysis  is  conducted  with  a  current- 
strength  of  0.1  to  0.5  ampere  and  a  potential-difference 
of  from  2.6  to  3.6  volts.  The  operation  requires  from  3  to 
4  hours,  and  the  end  of  the  precipitation  can  be  determined 
with  hydrogen  sulphide.  When  completed,  the  cathode 
is  removed  quickly,  washed  with  water,  alcohol,  and  ether, 
dried  as  before,  and  allowed  to  stand  in  a  desiccator  for  15 
minutes  before  weighing.  For  a  second  experiment  the 
cathode  can  be  cleaned  by  treating  with  almost  concentrated 
hydrochloric  acid,  and  is  washed  with  water,  etc.,  as  in  the 
first  case. 

Paweck  also  succeeded  in  precipitating  the  zinc  quan- 
titatively on  unamalgamated  brass- wire  gauze  cathodes. 
The  solution  in  this  case  consisted  of  the  zinc  salt  as  sul- 
phate, 14  g  sodium  or  potassium  sulphate  and  three  drops 
of  concentrated  sulphuric  acid.  The  circuit  was  so  ad- 
justed that  the  current  began  to  pass  as  soon  as  the  cathode 
was  introduced  into  the  solution,  and  the  washing  at  the 
end  was  carried  out  without  interrupting  the  current.  A 
potential-difference  of  3.6  volts  was  employed. 

Smith  has  determined  the  following  conditions  as  suitable 
for  the  determination  of  zinc  by  the  method  originally  pro- 
posed by  Riche. 


ZINC.  169 

To  the  solution  of  the  zinc  salt  (potassium  zinc  sulphate 
equal  to  0.2002  g  Zn)  1  g  of  sodium  acetate  and  0.3  cc  of 
acetic  acid  (99%)  were  added.  The  volume  of  the  final 
solution  was  150  cc  and  the  electrolysis  was  conducted  at 
a  temperature  of  65°  in  silvered  dishes,  with  a  current  of 
ND100=0.36  to  0.70  ampere  and  a  potential-difference  of 
4  to  5  volts.  The  time  required  was  two  hours,  and  towards 
the  end  of  the  operation,  when  the  solution  appeared  to 
be  filled  with  small  gas  bubbles,  it  was  neutralised  with  am- 
monia. 

For  the  electrolytic  determination  of  zinc  Smith*  states 
that  he  prefers  the  method  suggested  by  Parodi  and  Mas- 
cazzini,  which  he  describes  as  follows:  To  a  solution  of 
the  element  (0.1-0.25  g  Zn)  as  sulphate  add  4  cc  of  a  solu- 
tion of  ammonium  acetate  (ordinary  laboratory  strength, 
presumably!),  20  cc  of  citric  acid,  and  dilute  to  200  cc  with 
water.  The  electrodes  are  then  introduced  into  the  liquid, 
their  distance  apart  being  not  more  than  a  few  millimeters. 
A  platinum  cone  is  used  as  cathode  and  the  current  should 
be  0.5  ampere  and  5.9-6.3  volts  at  ordinary  temperatures, 
or  0.5  ampere  and  4.8  to  5.2  volts  when  the  solution  is  warmed 
to  50-60°. 

According  to  Vortmann,  zinc  can  be  quantitatively  pre- 
cipitated from  alkaline  (NaOH)  solutions  containing  sodium 
potassium  tartrate  by  currents  of  ND100  =  0.3  to  0.6  ampere. 
This  behavior  of  zinc  is  important  chiefly  in  separations 
(see  Cobalt-Zinc,  Zinc-Nickel). 

*  Electro-Chemical  Analysis  (1902),  p.  84. 


170  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


MANGANESE. 

LITERATURE  I 

Luckow,  Zeit.  f.  anal.  Chem.,  8,  24  (1869). 

Riche,  Ann.  d.  Chim.  et  Phys.,  [5],  13  508  (1878). 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  17  (1880). 

Schucht,  ibid.,  22,  493  (1883). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  (1881). 

Moore,  Chem.  News,  53,  209  (1886). 

Smith  and  Frankel,  Journ.  Anal.  Chem.,  3,  385  (1889) ; 

Chem.  News,  60,  262  (1889). 
Brand,  Zeit.  f.  anal.  Chem.,  28,  581  (1889). 
Riidorff,  Zeit.  f.  angew.  Chem.,  15,  6  (1892). 
Warwick,  Zeit.  f.  anorg.  Chem:,  i,  285  (1892). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Groger,  Zeit.  f.  angew.  Chem.,  p.  253  (1895). 

fingels,  Zeit.  f.  Elektrochemie,  413  (1895-96);  286,  305  (1896-97). 
Kaeppel,  Zeit.  f.  anorg.  Chem.,  16,  268  (1898). 

From  the  results  of  experience  in  the  Aachen  laboratory, 
none  of  the  methods  long  in  use  are  applicable  for  the  direct 
quantitative  determination  of  this  metal  as  peroxide.  It 
is  generally  assumed  that  the  peroxide  when  dried  at  about 
68°  has  the  composition  Mn02.H2O,  an  assumption  which 
the  author  cannot  confirm.  If  the  attempt  be  made  to  con- 
vert the  hydrated  peroxide  into  anhydrous  peroxide  by 
prolonged  drying  at  a  higher  temperature,  a  strongly  hygro- 
scopic substance  results  which  rapidly  increases  in  weight 
during  the  process  of  weighing.  It  is  therefore  necessary'  to 
convert  the  dried  peroxide  into  mangano-manganic  oxide 
by  ignition,  an  operation  conducted  with  ease  and  safety. 
After  determining  the  necessary  conditions  for  the  separa- 
tion of  large  quantities  of  lead  peroxide,  the  author  was  in- 
duced to  assume  that  manganese  behaved  similarly  to  lead. 
Investigation  proved,  however,  that  strong  inorganic  acids 
interfere  with  complete  precipitation,  and  even  make  it  im- 
possible. Of  the  organic  acids,  acetic  acid  alone  is  suitable, 


MANGANESE.  171 

although  the  precipitation  of  large  quantities,  even  when 
roughened  dishes  are  used,  cannot  be  successfully  carried 
out,  since  it  is  impossible  to  obtain  firmly  adhering  pre- 
cipitates. 

If  a  salt  other  than  the  acetate  is  at  hand,  it  is  best  to 
precipitate  the  manganese  as  dioxide  with  ammoniacal  hy- 
drogen peroxide.  The  precipitate  is  washed  thoroughly  and 
dissolved  in  5  cc  acetic  acid,  5  cc  hydrogen  peroxide  (4-5%), 
and  25  cc  water.  This  is  especially  necessary  when  the  man- 
ganese is  present  as  chloride  or  when  the  solution  contains 
other  chlorides.  Permanganic  acid  is  first  reduced  to  a 
manganous  salt.  In  acetic  acid  solutions,  even  when  rough- 
ened dishes  are  used,  the  maximum  quantity  of  manganese 
which  can  be  satisfactorily  determined  as  peroxide  is  only 
about  0.08  gram. 

CONDITIONS   FOR    ANALYSIS. 

Metal  present  as  acetate  or  sulphate. 

Substance  added:  25  cc  acetic  acid  (sp.  gr.  =  1.069). 

Total  volume  of  solution :  75  cc. 

Temperature:  50°  to  70°. 

Current-density  at  anode:  ND100  =  0.3  to  0.35  ampere. 

Potential-difference :  4.3  to  4.9  volts. 

Time  required:  3  hours. 

Roughened  dishes  should  be  used. 

A  rapid  and  complete  separation  was  secured  by  Engels, 
as  a  result  of  investigations  conducted  in  the  Aachen  labora- 
tory. The  method  is  as  follows:  1-2  g  of  the  manganese  salt 
is  dissolved  in  about  125  cc  of  water,  and  10  g  ammonium 
acetate  and  1.5-2  g  chrome  alum  are  also  added.  The  clear 
solution  is  then  electrolysed.  Chlorides  must  not  be  present, 
since  the  evolution  of  chlorine  interferes  with  the  separation 


172  QUANTITATIVE    ANALYSIS   BY    ELECTROLYSIS. 

of  the  manganese.     If  they  are  present,  the  manganese  is 
converted  into  acetate  as  described  above. 


CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  acetate  or  sulphate. 
Substance  added:   10  g  ammonium  acetate  and  1.5  to 
2  g  chrome  alum. 

Total  volume  of  solution :  125  cc. 

Temperature:  80°. 

Current-density  at  anode:  ND100  =  0.6  to  1.0  ampere. 

Potential-difference  between  electrodes:   2.8  to  4.0  volts. 

Time  required:  about  1J  hour. 

Roughened  dishes  must  be  used. 

EXPERIMENT. 

In  the  determinations  given  below,  10  g  ammonium  ace- 
tate and  1.5-2  g  chrome  alum  were  added  to  the  solution. 

Temp.Tim, 

1.1522g  0.6-0.5  2.8-3.1  80°  f  hr.  0.2235  19.39 

1.2554"  0.6-0.5  2.8-3.1  80°  "   "  0.2436  19.40 

1.2994"  0.6  3.  83°  "."  0.2520  19.39 

1.8099"  1.1  3.7-4.1  80°  ""  0.3513  19.40 

In  the  determination  of  manganese  in  the  salts  of  perman- 
ganic acid,  the  solution  of  the  latter  is  decomposed,  accord- 
ing to  Engels,  with  5  cc  acetic  acid  and  enough  hydrogen 
peroxide  to  completely  decolorise  it.  Since  the  presence  of 
even  small  quantities  of  hydrogen  peroxide  prevents  the  sepa- 
ration and  the  firm  adherence  of  the  precipitate,  the  excess 
of  hydrogen  peroxide  must  be  removed.  This  may  be  most 
easily  accomplished  by  the  addition  of  small  quantities  of 
chromic  acid,  until  further  addition  no  longer  causes  the  evo- 
lution of  gas;  generally  0.3-0.5  g  is  sufficient. 


MANGANESE.  173 


EXPERIMENT. 

50  cc  of  a  potassium  permanganate  solution  were  decom- 
posed with  5  cc  acetic  acid  and  10  cc  of  a  weak  solution  of 
hydrogen  peroxide.  The  excess  of  H20,  was  removed  with 
Cr03. 


current-density 

Potential. 

Time. 

Temp. 

Mn304. 

I. 

1  .  5    amp. 

2.  8  volts 

Ihr. 

85° 

0.1217g 

II. 

1.65    " 

3.15   " 

1    " 

85° 

0.1220" 

III. 

1.78    " 

3.4     " 

1    " 

80° 

0.1220" 

The  current-strength  available  varies  between  compara- 
tively wide  limits.  Weak  currents  also  give  rapid  and  satis- 
factory results. 

EXPERIMENT. 

Three  dishes,  each  containing  manganese  sulphate  solu- 
tion, 10  g  ammonium  acetate,  and  1  g  chrome  alum,  were  con- 
nected in  parallel,  and  the  current  from  a  thermopile  passed 
through.  The  potential  at  the  electrodes  at  the  beginning 
of  the  electrolysis  was  3.2  volts,  the  entire  current-strength 
1.5  amp.,  so  that  each  dish  received  about  0.4  amp.  The 
manganese  salt  used  contained  20.45%  Mn304. 

MOlUg*^^  Temp.          Time.  Found. 

1.1955  0.22  3.2         80°    2hrs.30min.    20.45% 

0.9009  0.22  3.2         80°    2"     "     "       20.44" 

1.2012  0.22  3.2         80°    2"     "     "       20.40" 

Since  manganese  separates  as  peroxide  from  a  cold  solution 
to  which  ammonium  acetate  has  been  added,  at  1.25  volts, 
and  when  warmed  to  80°  as  low  as  1-1.1  volts,  the  electroly- 
sis may  therefore  be  conducted  with  low  electromotive  forces. 
The  constancy  of  the  latter  may  be  assured  by  connecting  in 
shunt  (page  103).  The  lower  the  potential,  the  longer  the 


174  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

time  required  for  the  separation.  With  the  maximum  poten- 
tial of  1.8  volts  it  takes  from  4  to  5  hours.  For  the  firm  adher- 
ence of  the  precipitate  a  temperature  of  80°  is  essential. 

In  those  cases  (i.e.,  in  the  presence  of  silver)  where  the 
chrome  alum  produces  a  precipitate  in  the  solutions,  it  may 
be  replaced  by  10  cc  of  alcohol,  which  in  general  is  not  as 
satisfactory  as  the  chrome  alum  in  separating  the  manganese 
peroxide. 

When  alcohol  is  used,  the  electrolysis  is  conducted  at  a 
temperature  of  75-80°,  with  a  maximum  potential  of  2  volts, 
which  gives  a  current-density  ND100  =  about  0.15  amp.  Time 
required  for  the  electrolysis,  about  5  hours. 

Kaeppel  has  stated  that  by  the  addition  of  acetone  to 
solutions  of  manganese  sulphate  he  was  able  to  deposit 
quantities  of  manganese  dioxide  as  great  as  1.6  g  on  rough- 
ened anodes.  He  employed  solutions  containing  manganese 
equivalent  to  0.15  to  1.6  gram  of  manganese  dioxide  in  150 
cc,  and  added  from  1.5  to  10  cc  of  acetone,  depending  on 
the  amount  of  manganese  sulphate  present.  The  electrolysis 
was  conducted  at  a  temperature  of  from  50°  to  60°  and  re- 
quired from  2  to  5J  hours.  As  source  of  current  he  states 
that  he  used  accumulators  and  Cupron  elements,  giving  a 
potential-difference  of  4  to  4.25  volts  and  currents  of  from 
0.7  to  1.2  ampere,  directions  which  are  insufficient  to  permit 
a  repetition  of  his  experiments.  His  method  has  not  given 
.satisfactory  results  when  tried  in  the  author's  laboratory.* 

ALUMINIUM,    URANIUM,  CHROMIUM,    BERYLLIUM. 

If  a  solution  of  aluminium  ammonium  oxalate  containing 
^ammonium  oxalate  in  excess  is  submitted  to  the  action  of  the 
electric  current,  the  ammonium  oxalate  is  changed  into  car- 

*  Classen,  Ausgewahlte  Methoden,  p.  370. 


URANIUM.  175 

bonate,  and  the  aluminium  separates  as  hydroxide.  When 
the  oxalate  is  decomposed,  the  solution  is  heated  until  there 
is  only  a  faint  odor  of  ammonia,  the  hydroxide  filtered  off, 
washed  with  water,  and  converted  by  ignition  into  A1203. 

Uranium  is  acted  on  in  the  same  way  as  aluminium. 

Chromium  ammonium  oxalate  is  oxidised  by  the  current 
with  formation  of  ammonium  chromate.  To  determine  the 
chromic  acid,  the  ammonium  carbonate  is  decomposed  by 
boiling,  the  solution  acidified  with  acetic  acid,  and  the  chromic 
acid  determined  as  lead  or  barium  chromate. 

When  beryllium  ammonium  oxalate  is  subjected  to  elec- 
trolysis, the  beryllium  is  kept  in  solution  by  the  hydrogen 
ammonium  carbonate  produced,  provided  the  solution  is 
cold. 

The  behavior  of  aluminium,  chromium,  uranium,  and 
beryllium  can  be  made  use  of,  as  explained  later,  to  separate 
them  from  each  other  and  from  all  metals  which  separate 
from  their  double  oxalates  in  the  metallic  state  at  the  nega- 
tive electrode. 

URANIUM. 

LITERATURE  t 

Smith,  Am.  Chem.  Journ.,  i,  329  (1879). 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  18  (1880). 

Smith  and  Wallace,  Journ.  Am.  Chem.  Soc.,  20,  279  (1898). 

Kollock  and  Smith,  Journ.  Am.  Chem.  Soc.,  23,  607  (1901). 

Kern,  Journ.  Am.  Chem.  Soc.,  23,  685  (1901). 

According  to  Smith  and  Wallace  uranium  can  be  deter- 
mined by  the  electrolysis  of  a  solution  containing  free  acetic 
acid.  The  uranium  separates  on  the  cathode  as  yellow  uranic 
hydroxide,  which,  on  the  continued  action  of  the  current 
is  converted  into  black  hydrated  protosesquioxide.  When 
the  solution,  had  become  colorless,  the  current  was  inter- 


176  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

rupted,  the  precipitate  was  washed  with  dilute  acetic  acid  and 
boiling  water,  dried,  ignited  to  protosesquioxide,  and  weighed. 

To  a  solution  containing  urano-uranic  oxide  (Ur308  = 
0.1185  gin  10  cc),  0.5  cc  of  concentrated  acetic  acid  was  added 
and,  after  diluting  to  40  cc,  the  solution  was  electrolysed  at 
70°  with  a  current  of  ND100  =  0.18  ampere  and  a  potential- 
difference  of  3  volts.  The  precipitation  was  complete  in 
5  hours. 

Kollock  and  Smith  recommend  the  following  conditions: 
uranium  acetate  (U308  =  0.0986-0. 2295  g),  0.2  cc  acetic  acid 
(29%),  total  volume  of  solution  =  125  cc,  temperature  65-70°, 
ND100  =  0.05-0.55,  potential-difference  =  4.0-16.2  volts,  time 
4-6  hours. 

For  uranyl  nitrate  solutions,  the  same  authors  recom- 
mend the  following  conditions:  0.1222-0.1320  g  U308,  vol- 
ume of  solution  =  125  cc,  temp.  =  65-75°,  ND100  =  0.02-0.04 
ampere,  potential-difference  =  2. 0-4. 6  volts,  time  5-8  hours. 

COPPER. 

LITERATURE : 

Gibbs,  Zeit.  f.  anal.  Chem.,  3,  334  (1864). 

Hampe,  Berg-  u.  Hiittenm.  Ztg.,  21,  220  (1862);  find.,  25,  113  (1866). 

Luckow,  Journ.  f.  prak.  Chem.,  96,  259  (1865). 

Boisbaudran,  Bull.  Soc.  Chim.,  7,  468  (1867). 

Luckow,  Zeit.  f.  anal.  Chem.,  8,  23  (1869). 

Mansfeld.  O.  Berg-  u.  H.  Direktion,  Zeit.  f.  anal.  Chem.,  8,  23  (1869). 

Merrick,  Amer.  Chemist,  2,  136  (1871). 

Mansfeld.  O.  Berg-  u.  H.  Direktion,  Zeit.  f.  anal.  Chem.,  u,  1  (1872). 

Classen  and  v.  Reiss,  Zeit.  f.  anal.  Chem.,  14,  246  (1875). 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  299  (1876). 

Herpin,  Zeit.  f.  anal.  Chem.,  15,  335  (1876). 

Schweder,  Berg-  u.  Huttenm.  Ztg.,  36,  11,  21  (1877); 

Zeit.  f.  anal.  Chem.,  16,  345  (1877). 
Fresenius,  Zeit.  f.  anal.  Chem.,  16,  339  (1877). 
Ohl,  Zeit.  f.  anal.  Chem.,  18,  523  (1879). 
Luckow,  Zeit.  f.  anal.  Chem.,  19,  1  (1880). 


COPPER.  177 

Classen,  Ber.  deutsch.  chem.  Ges.,  14,  1622,  1627  (1881). 

Mackintosh,  Am.  Chem.  Journ.,  3,  354  (1881). 

Riche,  Zeit.  f.  anal.  Chem.,  21,  116  (1882). 

Foote,  Am.  Chem.  Journ.,  6,  333  (1884). 

Moore,  Chem.  News,  53,  209  (1886). 

Riidorff,  Ber.  deutsch.  chem.  Ges.,  21,  1888  (1888). 

Classen,  Ber.  deutsch.  chem.  Ges.,  21,  2898  (1888). 

Oettel,  Zeit.  f.  anal.  Chem.,  27,  15  (1888). 

Smith,  Am.  Chem.  Journ.,  12,  329  (1890). 

Croasdale,  Journ.  Anal.  Chem.,  5,  133  (1891). 

Regelsberger,  Zeit.  f.  angew.  Chemie,  2,  473  (1891). 

Meeker,  Journ.  Anal.  Chem.,  6,  267  (1892). 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  5  (1892). 

Warwick,  Zeit.  f.  anorg.  Chem.,  i,  285  (1892). 

Schmucker,  Journ.  Anal.  Chem.,  7,  252  (1893). 

Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  4,  267  (1893). 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 

Oettel,  Chemiker-Zeitung,  18,  47,  879  (1894). 

Wagner,  Zeit.  f.  Elektrochemie,  2,  613  (1895-96). 

Heidenreich,  Ber.  deutsch.  chem.  Ges.,  29,  1585  (1896). 

Hollard,  Compt.  rend.,  123,  1003  (1896). 

Forster  and  Seidel,  Zeit.  f.  anorg.  Chem.,  14,  106  (1897). 

Revay,  Zeit.  f.  Elektrochem.,  4,  313  (1897-98). 

Head,  Trans.  Am.  Inst.  Mining  Engineers  (1898). 

Ullmann,  Chem.  Ztg.,  22,  808  (1898). 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 

Gooch  and  Medway,  Am.  Journ.  of  Science,  April,  1903. 

If  copper  be  reduced  from  a  solution  containing  an  excess 
of  ammonium  oxalate,  it  is  not  always  possible  to  obtain  the 
metal  in  a  compact  form.  For  this  reason  the  author,  as 
long  ago  as  1888,*  began  experiments  on  the  determination 

of  this  metal  from  a  solution  of  the  acid  double  oxalate. 

• 

Further  experiments  in  this  direction  have  shown  that  co- 
herent, bright-red  copper  precipitates  can  be  obtained  when 
copper  is  reduced  from  such  solutions  at  a  temperature  of 
about  80°.  The  solution  containing  the  copper  is  treated 
with  a  cold  saturated  solution  of  ammonium  oxalate,  heated 
*  Ber.  deutsch.  chem.  Ges.,  21,  2898  (1888). 


178          QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

,as  dfrected,  and  at  first  electrolysed  for  a  few  minutes  with- 
out the  addition  of  oxalic  acid.  A  cold  saturated  oxalic  acid 
'solution  is  then  run  in  from  a  burette.  The  method  of  pro- 
cedure here  is  similar  to  that  described  under  Zinc  (p.  165). 

In  the  analysis  of  substances  low  in  copper,  the  solution 
>rtay  be  made  acid  at  the  start ;  in  concentrated  solutions,  on 
the  contrary,  the  electrolysis  must  be.  conducted  in  solutions 
-  which  are  as  nearly  neutral  as  possible,  since  otherwise  diffi- 
cultly soluble  oxalate  of  copper  will  separate  out,  owing  to 
the  free  oxalic  acid  present.  The  end  of  the  reaction  is  de- 
termined by  testing  with  potassium  ferrocyanide  a  small  por- 
tion of  the  solution  strongly  acidified  with  hydrochloric  acid. 
The  precipitate  must  be  washed  without  stopping  the  current. 
The  metal  is  dried  in  an  air-bath  after  treating  with  water 
and  alcohol. 

The  precipitated  copper  has  a  bright-red  color,  adheres 
firmly  to  the  dish,  and  has  little  resemblance  to  the  copper 
precipitated  from  nitric  acid  solutions  (see  below).  The 
chief  advantage  of  this  method  is  the  rapidity  with  which  it 
may  be  conducted. 

CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  sulphate. 

Substance  added:    4  g   ammonium  oxalate,  oxalic  acid 
*  as  described  above. 

Total  volume  of  solution :   120  cc. 
Temperature:  80°.  — 

jfliirrent-density  at  cathode:   ND100  =  1.0  ampere*. 
Potential-difference:  2.5  to  3.2  volts. 
Time  required :  2  hours. 

EXPERIMENT. 

Used  1  g  copper  sulphate,  4  g  ammonium  oxalate,  oxalic 
acid  as  described  above,  120  cc  solution. 


COPPER.  179 


Current-density,       poei,  Temp.        Time.          Taken.  Found. 

NDj  00  =  Amperes.       Volts. 

1.0-0.8         2.8-3.2  80°       2   hr.      0.2529g         0.2531g 

0.45-0.35       2.5-2.8  80°       2$  "       0.2529"         0.2528" 

Copper  precipitate  bright  red. 


As  has  been  observed  by  Luckow,  copper  may  also  be 
precipitated  from  a  solution  to  which  nitric  acid  has  been 
added. 

The  reduction  of  copper  from  a  nitric  acid  solution  de- 
pends upon  the  presence  of  a  certain  quantity  of  nitric  acid 
and  the  absence  of  chlorides.  To  about  200  cc  of  solution, 
containing  the  copper  as  sulphate,  20  cc  of  nitric  acid  *  (sp. 
gr.  =  1.21)  are  added  and  the  liquid  is  subjected  to  electrolysis. 
The  end  of  the  reaction  is  determined  with  ammonia. 

The  presence  of  chlorides  is  to  be  avoided.  In  the  pres- 
ence of  antimony,  arsenic,  mercury,  silver,  tin,  and  bismuth, 
traces  of  these  metals  come  down  with  the  copper,  especially 
when  strong  current-densities  are  employed.  In  the  presence 
of  these  elements  it  is  best  to  conduct  the  electrolysis  with 
currents  of  from  0.2  to  0.3  ampere,  the  maximum  quantity 
of  10%  by  volume  of  concentrated  nitric  acid  being  present 
in  the  solution.  Iron,  cobalt,  nickel,  cadmium,  manganese, 
and  zinc  can  be  separated  readily  from  copper  by  this  method. 

According  to  the  researches  of  Schroder  large  quantities 
of  iron  are  detrimental,  since  a  secondary  reaction  may  take 
place  between  the  ferric  salt  formed  and  the  precipitated 
copper,  which  causes  the  copper  to  redissolve. 

Copper  separates  in   a   crystalline  form  from  solutions 


*  Such  a  large  quantity  of  nitric  acid  is  required  only  when  the  copper 
is  to  be  separated  from  other  metals  present  in  the  same  solution.  When 
no  other  metal  is  present  2  to  3  per  cent,  by  volume  of  concentrated  acid 
is  sufficient. 


180  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

warmed  to  50-60°;    it  is  moreover  impossible  to  separate 
the  last  traces  of  copper  at  this  temperature. 

CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  nitrate  or  sulphate. 

Substance  added :  5%  by  volume  dilute  nitric  acid. 

Total  volume  of  solution:  120  to  150  cc. 

Temperature:   20°  to  30°. 

Current-density  at  cathode:  ND100  =  0.5  to  1.0  ampere 
(the  latter  only  when  no  other  metal  than  copper  is  present 
in  the  solution). 

Potential-difference  between  electrodes:   2.2  to  2.5  volts. 

Time  required:  4  to  5  hours.  By  continuously  stirring 
the  solution  the  operation  is  hastened. 

EXPERIMENT. 

Used  about  1  g  copper  sulphate  and  5%  by  volume  nitric 
acid.  Entire  volume  of  liquid  120  cc. 


Current  -density 
ND100,  Amperes. 

Electrode 
Potential, 
Volts. 

Temp. 

Time. 

Taken 

Found. 

1.1-1, 

,0 

2. 

2  -2. 

5 

25-30° 

5hr. 

0. 

2495  g 

Cu 

0. 

2490  g.  Cu 

1.0-0 

.95 

2. 

25-2 

.3 

30-32° 

5  " 

0. 

2510  " 

It 

0 

.2505"    " 

A  solution  containing  free  nitric  acid  may  also  be  used 
for  separating  such  metals  as  are  not  reduced  in  the  presence 
of  this  acid,  or  which  are  set  free  at  the  positive  electrode  in 
the  form  of  peroxides.  In  such  cases,  however,  it  must  be 
kept  in  mind  that  the  nitric  acid  is  gradually  converted  into 
ammonia,  on  account  of  which,  after  the  current  has  acted 
for  some  time,  nitric  acid  must  be  occasionally  added. 

For  the  determination  of  copper,  employing  the  apparatus 
described  on  p.  124,  Gooch  and  Medway  used  a  solution  con- 


COPPER.  181 

taming  0.0651-0.2548  g  of  copper  as  sulphate,  to  which  6-7 
drops  of  dilute  sulphuric  acid  (1:4)  were  added.  The  total 
volume  of  the  solution  was  about  50  cc,  and  the  electrolysis 
was  conducted  with  a  current  of  from  0.8  to  4  amperes 
(equivalent  to  ND100  =  2 .7- 1 3 .3  amperes) .  The  tune  required 
for  the  complete  precipitation  of  the  copper  was  from  10  to 
25  minutes. 

The  same  investigators  also  used  a  solution  containing 
0.0651  g  of  copper  to  which  from  6  to  9  drops  of  dilute 
nitric  acid  (1:4)  had  been  added.  The  volume  of  the  solu- 
tion was  50  cc  and  the  precipitation  of  the  copper  was 
conducted  with  a  current  of  0.8-1.8  ampere,  equal  to 
ND100  =  2.7-6  amperes.  The  time  required  was  from  20  to 
35  minutes.  From  tests  of  this  method  made  by  the  trans- 
lator it  would  appear  that  the  precipitation  of  copper  from 
a  nitric  acid  solution  is  more  satisfactory  and  equally  rapid 
when  the  quantity  of  dilute  nitric  acid  (1:1)  added  to  the 
solution  is  equal  to  about  5%  of  the  final  volume.  Under 
these  conditions  the  conductivity  of  the  solution  is  so  high 
that, the  temperature  of  the  electrolyte  remains  low  through- 
out the  electrolysis.  As  stated  elsewhere  in  the  text,  the 
separation  of  copper  from  a  nitric  acid  solution  proceeds 
most  satisfactorily  when  the  solution  is  cold. 

Copper  may  be  separated  from  a  solution  containing  am- 
monium oxalate  or  one  containing  free  nitric  acid,  in  the 
presence  of  small  quantities  of  antimony  and  arsenic.  If, 
however,  the  amounts  of  the  latter  are  considerable,  then, 
after  continued  action  of  the  current,  antimony  and  arsenic 
are  deposited  upon  the  copper,  causing  the  negative  electrode 
to  appear  more  or  less  dark-colored.  In  order  to  determine 
the  copper  hi  such  cases,  the  dried  electrode  is  ignited  for  a 
short  time,  as  a  result  of  which  the  copper  is  oxidised  and 
the  antimony  and  arsenic  are  driven  off.  The  residue  of 


182     QUANTITATIVE  ANALYSIS  BY  ELECTROLYSIS. 

oxide  is  dissolved  in  nitric  acid  and  again  submitted  to  elec- 
trolysis.* 

In  general  the  presence  of  chlorides  causes  the  copper  to 
separate  in  a  spongy  condition.  To  avert  this  action  and  to 
secure  an  adherent  precipitate,  Riidorff  adds  2-3  g  ammonium 
nitrate  and  20  cc  ammonia  (sp.  gr.  0.96),  dilutes  with  water 
to  100  cc,  and  electrolyses  this  solution.  At  the  close  of  the 
reduction  the  solution  is  acidified  with  dilute  acetic  acid,  the 
dish  filled  to  overflowing  with  water,  emptied,  shaken  to  re- 
move the  last  drops  of  water,  and  dried  at  100°  in  the  air- 
bath. 

In  the  laboratory  of  the  Munich  Polytechnic  Institute 
the  preceding  method  is  carried  out  under  the  following  con- 
ditions :  Ammonia  is  added  in  slight  excess  until  the  precipi- 
tate which  at  first  appears  is  redissolved.  Then  20-25  cc 
ammonia,  sp.  gr.  0.96,  are  added,  in  case  not  more  than  0.5  g 
copper  is  present. f  In  this  solution  3-5  g  ammonium  nitrate 
are  dissolved,  it  is  diluted  to  100  cc,  and  the  electrolysis  is 
conducted  with  a  current  of  ND100  =  2  amperes.  The  precipi- 
tate must  be  washed  without  interrupting  the  current. 

Oettel,  who  also  carried  out  experiments  on  the  quantita- 
tive determination  of  copper  from  ammoniacal  solutions, 
found  that,  by  the  addition  of  ammonium  nitrate,  0.2-0.25  g 
of  copper  sulphate  was  quantitatively  reduced  in  6-8  hours 
at  ordinary  temperatures.  The  results  of  his  investigation 
are: 

"1.  That  copper  can  be  separated  in  a  compact  form 
from  weakly  ammoniacal  solutions  containing  ammonium 
nitrate,  by  currents  of  ND100  =  0.07-0. 27  ampere.  With  too 
little  ammonium  nitrate,  as  well  as  in  the  presence  of  large 

*  Mansfeld'sche  Hiittendirektion. 

t  If  as  much  as  1  g  Cu  is  present,  the  quantity  of  ammonia  is  increased 
to  30-35  cc. 


COPPER.  183 

quantities  of  free  ammonia,  the  precipitate  shows  a  tendency 
to  a  spongy  structure. 

"2.  The  highest  concentration  of  the  solution  is  0.8  g 
copper  per  100  sq.  cm  electrode  surface,  with  the  employ- 
ment of  a  wire-shaped  positive  electrode. 

"3.  The  presence  of  chlorine,  zinc,  arsenic,  and  small 
quantities  of  antimony  is  without  detrimental  action;  when 
the  solution  contains  lead,  bismuth,  mercury,  cadmium,  or 
nickel,  the  results  of  the  determinations  are  somewhat  too 
high." 

E.  F.  Smith  has  proposed  the  determination  of  copper 
in  solutions  containing  hydrogen  disodium  phosphate  and 
free  phosphoric  acid.  The  proper  conditions,  according  to 
Fernberger  and  Smith,  are  the  following: 

Metal  present  as  sulphate  (  =  1293  g  Cu);  substance 
added:  20  cc  of  a  solution  of  hydrogen  disodium  phosphate 
(sp.  gr.  =  1.0358)  and  5  cc  phosphoric  acid  (sp.  gr.  =  1.347); 
temperature:  54-64°;  total  volume:  225  cc;  ND100  = 
0.035-0.068  amperes;  potential-difference:  2.2-2.6  volts; 
time:  6-7  hours. 

A  rapid  and  accurate  method  for  the  determination 
of  copper  has  been  worked  out  by  Carl  Engels  in  the  Aachen 
laboratory.  This  method  has  the  advantage  over  the  use 
of  nitric  acid  solutions  that  it  can  be  more  rapidly  performed, 
and  that,  in  separations,  it  also  dispenses  with  the  tedious 
conversion  of  the  nitrates  into  sulphates.  This  method 
is  based  upon  the  addition  of  urea. 

The  separation  of  copper  from  solutions  containing  sul- 
phuric acid  is  possible  also  if  hydroxylamine  is  added.  The 
method  is  as  follows: 

If  the  separation  is  to  be  carried  out  with  weak  currents,, 
say  during  the  night,  the  addition  of  2  cc  concentrated  sul- 
phuric acid  and  about  J  g  hydroxylamine  sulphate  is  recom- 


184  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

mended.  A  fine  crystalline  precipitate  and  absolutely  accu- 
rate results  are  obtained  with  a  current-strength  of  ND100  = 
0.08-0.18  ampere.  The  potential  at  the  poles  of  a  shunt  cir- 
cuit was  1.8-2.2  volts;  after  connecting  the  dish  the  potential 
sank  to  1.1-1.3  volts,  with  a  current  of  0.1-0.2  amp. 

EXPERIMENT. 

Taken  Current -density  Potential,         rp-  Found,  T>     r«     *  * 

CuS04.5H30.  ND100.  Volts.  Cu.  Per  Cent  * 

1.0130g  0.1     amp.         1.1  Night.          0.2574  25.41 

1.7065"  0.12     "  1.3  "  0.4335  25.40 

1.1893"  0.1        "  1.2  "  0.3021  25.41 

If  stronger  currents  are  used,  the  amount  of  sulphuric 
acid  must  be  increased.  10-15  cc  of  cone,  sulphuric  acid 
are  poured  into  the  solution  of  the  salt,  it  is  diluted  to  150 
cc,  and  1  g  hydroxylamine  sulphate  is  added.  If  0.3-0.5 
g  Cu  is  present,  with  a  current-strength  of  ND100=1  amp., 
the  separation  is  finished  in  1J  to  2  hours.  The  condition  of 
the  precipitated  copper  is  much  better  and  much  more  suited 
for  quantitative  determination  than  the  copper  obtained 
under  similar  conditions  without  the  addition  of  hydroxyl- 
amine. 

Urea  exerts  a  far  more  satisfactory  action  than  hydroxyl- 
amine upon  the  separation  of  copper  from  solutions  contain- 
ing sulphuric  acid.  With  a  current-strength  of  ND100  =  1 
ampere,  not  the  slightest  tendency  toward  a  spongy  separa- 
tion is  exhibited,  but  a  bright-red,  crystalline  coating  is 
obtained  on  the  negative  electrode.  The  analysis,  with  the 
stated  current-density,  is  completed  in  1J  hours. 

10-15  cc  concentrated  sulphuric  acid  and  1  g  urea  are 
added  to  the  solution  of  the  copper,  which  is  then  diluted  to 
150  cc. 

*  [Theory  25.33%  Cu.] 


COPPER.  185 


CONDITIONS   FOR   EXPERIMENT. 

Temperature  of  liquid:   Most  suitable,  60-70°. 
Potential-difference:  2.7-3.1  volts. 
Current-density:  ND100  =  0.8-1  amp. 
Time:   1J  hours. 

EXPERIMENT. 

Used  copper  sulphate  containing  28.08%  Cu. 

QuantgSubst.,  C^nt-denaity  Potential.       Temp.  Time.  Foun(L 

1.1364  1.05  3.1  25°  Ihr.lSm.  25.09% 
0.9671  1.2  3.1  55°  1  "  15  "  25.09  " 
1.3972  0.75  2.7  65°  1  "  45"  25.09  " 

The  current  may  be  interrupted  in  washing  the  precipitate. 
The  separated  copper  contains  traces  of  carbon,  and  also 
platinum  which  dissolves  from  the  anode.  These  admixtures 
can  be  determined  by  dissolving  the  copper  in  dilute  nitric 
acid  (1  : 10).  A  thin  dark  coating  remains  on  the  dish, 
which  may  be  washed  with  water,  but  not  with  alcohol,  with- 
out becoming  loosened.  The  weight  of  the  dish,  determined 
after  washing  and  drying  in  the  air-bath,  is  used  as  a  basis 
for  calculating  the  weight  of  the  separated  copper. 

With  weaker  currents  the  length  of  time  required  is  of 
course  greater.  With  a  current-density  of  ND100  =  0.2  am- 
pere, the  precipitation  of  from  0.3  to  0.4  g  Cu  is  completed 
in  3J-4  hours.  It  is  desirable  in  this  case  to  add  less  sul- 
phuric acid  to  the  solution;  5  cc  cone.  H2S04  to  each  150  cc, 
is  the  proper  proportion. 

Four  dishes  were  connected  in  parallel,  and  for  every  150 
cc  of  solution  of  the  copper  salt  which  they  contained  1  g 
urea  and  5  cc  cone.  H2S04  were  added.  The  four  electroly- 
ses were  then  conducted  hi  the  cold,  with  the  current  from 
a  thermopile.  The  entire  current-strength  was  ND100  =  0.8 


186  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

ampere,  so  that  each  dish  received  a  current  of  ND100  =  0.2 
ampere.  The  analyses  were  completed  in  4  hours.  The  per 
cent,  of  copper  in  the  salt  used  was  25.08. 

Used  CuSO4.5HaO.  Found  Cu.  Found  #. 

l.OlOlg.  0.2533g.  25.07 

1.0815"  0.2709"  25.05 

1.0320"  0.2589"  25.08 

1.0111"  0.2535"  25.07 

BISMUTH. 

LITERATURE. 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  16  (1880). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  (1881). 

Thomas  and  Smith,  Am.  Chem.  Journ.,  5,  114  (1883). 

Schucht,  Zeit.  f.  anal.  Chem.,  22,  492  (1883). 

Wieland,  Ber.  deutsch.  chem.  Ges.,  17,  1612  (1884). 

Moore,  Chem.  News,  53,  209  (1886). 

Smith  and  Knerr,  Am.  Chem.  Journ.,  8,  206  (1886). 

Eliasberg,  Ber.  deutsch.  chem.  Ges.,  19,  326  (1888). 

Brand,  Zeit.  f.  anal.  Chem.,  28,  596  (1889). 

Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749  (1891). 

omith  and  Frankel,  Am.  Chem.  Journ.,  12,  428  (1891). 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  199  (1892). 

Smith  and  Saltar,  Zeit.  f.  anorg.  Chem.,  3,  418  (1893). 

Smith  and  Moyer,  Journ.  of  the  Am.  Chem.  Soc.,  15,  28,  101  (1893). 

Schmucker,  Zeit.  f.  anorg.  Chem.,  5,  199  (1894). 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Balachowsky,  Compt.  rend.,  131,  179  (1900). 

Wimmenauer,  Zeit.  f.  anorg.  Chem.,  27,  1  (1901). 

Brunck,  Ber.  deutsch.  chem.  Ges.,  35,  1871  (1902). 

Kammerer,  Journ.  Am.  Chem.  Soc.,  25,  83  (1903). 

Until  recently  it  has  been  found  impossible  to  quanti- 
tatively precipitate  bismuth  in  a  compact,  metallic  form, 
since  it  has  a  strong  tendency  to  separate  in  a  more  or  less 
spongy  state  from  all  of  its  compounds. 

In  an  article  published  in  1903,  Kammerer  (loc.  cit.) 
states  that  this  element  can  be  satisfactorily  determined 
under  the  following  conditions : 


CADMIUM.  187 

Metal:  0.10  to  0.15  g  Bi  dissolved  in  1  cc  nitric  acid 
(sp.gr.  =  1.42). 

Substance  added:  2  cc  sulphuric  acid  (sp.  gr.  1.84).;  1  g 
potassium  sulphate. 

Total  volume :  150  cc. 

Temperature:  45°  to  50°. 

Current-strength:  ND100  =  0.02  ampere. 

Potential-difference:  1.8  volt. 

Time:  8  to  9  hours. 

The  end  of  the  precipitation  is  determined  with  ammo- 
nium sulphide,  or  by  adding  water  to  the  contents  of  the 
dish  and  observing  whether  a  dark  ring  appears  above  the 
deposited  metal.  During  the  electrolysis  the  electrolytic 
vessel  should  be  tightly  covered  to  prevent  evaporation. 
The  precipitate  should  be  washed  without  interrupting  the 
current,  first  with  hot  water,  then  with  a  mixture  of  1  part 
alcohol  and  2  parts  ether,  and  finally  with  pure  anhydrous 
ether. 

CADMIUM. 

LITERATURE : 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  303  (1876). 

Smith,  Pr.  Am.  Phil.  Soc.  (1878). 

Clarke,  Zeit.  f.  anal.  Chem.,  18,  104  (1879).    - 

Beilstein  and  Jawein,  Ber.  deutsch.  chem.  Ges.,  12,  759  (1879).    -^ 

Smith,  Am.  Chem.  Journ.,  2,  43  (1880). 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  16  (1880). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1628  (1881).     -I 

Moore,  Chem.  News,  53,  209  (1886). 

Brand,  Zeit.  f.  anal.  Chem.,  28,  581  (1889).  - 

Smith,  Am.  Chem.  Journ.,  12,  329  (1890). 

Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749  (1891).  J 

Pviidorff,  Zeit.  f.  angew.  Chem.,  p.  197  (1892). 

Smith  and  Wallace,  Ber.  deutsch.  chem.  Ges.,  25,  779  (1892). 

Warwick,  Zeit.  f.  anorg.  Chem.,  i,  258,  291  (1892). 

Smith  and  Muhr,  Journ.  Anal.  Chem.,  7,  189  (1893). 


188  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 

Heidenreich,  ibid.,  29,  1586  (1896). 

Hardin,  Journ.  Am.  Chem.  Soc.,  18,  990  (1896). 

Avery  and  Dales,  Journ.  Am.  Chem.  Soc.,  19,  380  (1897). 

Wallace  and  Smith,  Journ.  Am.  Chem.  Soc.,  19,  870  (1897). 

Rimbach,  Zeit.  f.  anal.  Chem.,  37,  284  (1898). 

Smith  and  Wallace,  Journ.  Am.  Chem.  Soc.,  20,  279  (1898). 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Balachowsky,  Compt.  rend.,  131,  384  (1900). 

Miller  and  Page,  Chem.  News,  84,  312  (1901). 

The  separation  of  this  metal  in  a  compact  form,  with  a 
bright  metallic  luster,  can  be  accomplished  *  by  the  electroly- 
sis of  a  solution  of  the  double  oxalate  which  is  kept  acid  by  the 
addition  of  a  cold  saturated  solution  of  oxalic  acid  during 
the  progress  of  the  operation  (see  under  Zinc). 

To  prepare  the  double  salt,  the  cadmium  compound  is 
dissolved  in  20-25  cc  water  by  warming  in  a  platinum  dish; 
a  hot  solution,  which  should  be  previously  filtered,  of  10  g 
ammonium  oxalate  in  80-100  cc  water  is  added  and  the  solu- 
tion is  electrolysed.  As  soon  as  the  action  of  the  current  has 
begun,  several  cubic  centimeters  of  oxalic  acid  are  poured 
upon  the  watch-glass  covering  the  dish  and  the  liquid  is  kept 
weakly  acid  during  the  electrolysis. 

CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  sulphate. 

Substance  added:  10  g  ammonium  oxalate,  oxalic  acid 
as  described  above. 

Total  volume  of  solution :   120  cc. 

Temperature:  70°  to  75°. 

Current-density  at  cathode:  ND100  =  0.5  to  1.0  ampere. 

Potential-difference:  3.0  to  3.4  volts. 

*  The  condition  of  the  precipitated  cadmium,  when  the  directions 
are  closely  followed,  depends  upon  the  absolute  cleanliness  of  the  surface 
of  the  cathode. 


CADMIUM.  189 

Time  required:  about  3  hours.  The  maximum  quantity 
of  metal  which  can  be  precipitated  is  about  0.15  g.  Polished 
dishes  give  the  best  results. 

The  end  of  the  reaction  is  determined  with  hydrogen 
sulphide,  by  testing  a  small  portion  of  tneifcption  acidified 
with  hydrochloric  acid.  The  metal  must  be  washed  without 
interrupting  the  current.  (Method  of  the  author.) 

Smith  and  Luckow  recommend  the  precipitation  of  cad- 
mium from  a  solution  of  the  chloride  or  sulphate,  which  has 
been  saturated  with  sodium  acetate.  Eliasberg,  who  tested 
this  method  in  the  Aachen  laboratory,  found  that  the  reduc- 
tion took  place  readily  when  the  solution,  of  about  100  cc 
volume,  was  treated  \\ith  about  3  g  sodium  acetate  and  a  few 
drops  of  acetic  acid,  and  the  electrolysis  was  carried  out  at  a 
temperature  of  40-50°. 

In  the  laboratory  of  the  Munich  Polytechnic  Institute 
the  foregoing  method  is  practiced  as  follows:  The  solution, 
neutralised  if  necessary,  containing  not  more  than  0.5  g  cad- 
mium, is  treated  with  3  g  sodium  acetate  and  made  weakly 
acid  with  acetic  acid.  The  solution  is  warmed  to  45°  and 
electrolysed  with  a  current  of  ND  100  =  0.02-0.07  '  ampere. 
The  metal  is  washed  without  interrupting  the  current,  and 
quickly  dried  at  100°. 

During  the  electrolysis  the  solution  should  not  be  warmed 
above  50°,  on  account  of  the  formation  of  basic  salts.  Cad- 
mium is  only  partly  precipitated  from  solutions  strongly 
acidified  with  acetic  acid.  By  this  method  0.2  g  of  cadmium 
may  be  separated  in  about  five  hours.  The  presence  of 
nitrates  is  detrimental. 

According  to  Beilstein  and  Jawein,  the  determination 
of  cadmium  may  be  conducted  from  a  solution  of  the  double 
salt  with  potassium  cyanide.  A  solution  of  potassium  cya- 
nide is  added  to  the  solution  of  the  cadmium  salt  until 


190  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  precipitate  at  first  formed  is  redissolved  and  a  slight 
excess  of  .potassium  cyanide  is  present.  This  solution  is 
diluted  to  150  cc,  and  electrolysed  at  ordinary  temperatures 
with  a  current  of  ND100  =  0.5  ampere  and  a  potential-differ- 
ence of  4.7-5  volts.  Time  required  6-7  hours.  The  com- 
pletion of  the  precipitation  of  the  cadmium  is  determined 
by  removing  a  small  quantity  of  the  solution,  adding  a 
slight  excess  of  sulphuric  acid,  boiling  to  expel  the  hydro- 
cyanic acid,  and  testing  with  hydrogen  sulphide.  (Classen, 
Ausgewahlte  Methoden,  p.  112.) 

The  precipitation  of  cadmium  from  a  potassium  cyanide 
solution  is  described  by  Kollock  as  follows : 

Ten  cubic  centimeters  of  a  solution  of  cadmium  sulphate, 
containing  0.1659  g  of  Cd,  was  used,  and  to  this  1  g  of  potas- 
sium cyanide  was  added.  The  solution  was  diluted  to  a 
volume  of  125  cc,  and  electrolysed  at  60°  with  a  current  of 
ND100  =  0.04  to  0.06  ampere  and  a  potential-difference  of 
from  2.9  to  3.2  volts.  The  cadmium  was  completely  precipi- 
tated in  about  5  hours. 

Vortmann  has  suggested  the  determination  of  cadmium  by 
a  method  similar  to  that  used  for  the  determination  of  bismuth 
and  zinc,  by  precipitation  from  a  solution  of  the  ammonium 
double  salt  in  the  form  of  amalgam. 

A  method  of  determination  proposed  by  E.  F.  Smith  has 
been  described  by  Wallace  and  Smith  as  follows : 

0.1329  g  of  cadmium  oxide  was  dissolved  in  acetic  acid, 
the  solution  was  evaporated  to  dryness,  and  the  residue  was 
dissolved  in  30  cc  of  water.  This  solution  was  warmed  to 
50°  and  electrolysed  with  a  current  of  ND100  =  0.06  ampere 
and  a  potential-difference  of  3.5  volts.  The  metal  was  com- 
pletely precipitated  in  four  hours.  By  adding  1  g  of  ammo- 
nium acetate  after  the  electrolysis  had  proceeded  for  about  1 
hour  the  time  required  for  complete  precipitation  was  short- 


LEAD.  191 

ened.  The  deposit  should  be  washed  without  interrupting 
the  current. 

The  same  authors  also  describe  the  following  experiment: 
0.1270  g  of  cadmium  oxide  was  dissolved  in  2  cc  of  sulphuric 
acid  (sp.  gr.  1.09),  the  solution  was  diluted  to  30  cc,  and 
electrolysed  for  a  period  of  4J  hours  at  50°  wi.th  a  current 
of  ND100  =  0.15  ampere  and  a  potential-difference  of  2. 5  volts. 
The  deposited  cadmium  was  washed  without  interrupting 
the  current,  and  was  in  the  form  of  an  adherent  coating 
which  gave  excellent  results. 

Another  method  has  been  proposed  by  Smith  which 
depends  upon  the  precipitation  of  cadmium  from  a  solution 
containing  phosphates  and  free  phosphoric  acid.  It  is  best 
illustrated  by  the  following  experiment  described  by  Smith 
and  Wallace: 

To  ten  cubic  centimeters  of  a  solution  of  cadmium  sul- 
phate, containing  0.1656  g  cadmium,  30  cc  of  a  solution  of 
disodium  hydrogen  phosphate  (sp.  gr.  1.0358)  and  1.5  cc 
of  phosphoric  acid  (sp.  gr.  1.347)  were  added,  the  solution 
was  diluted  to  100  cc,  warmed  to  50°  and  electrolysed  with 
a  current  of  ND100  =  0.06  ampere  and  a  potential-difference 
of  3  volts.  At  the  expiration  of  4  hours  the  current  was 
increased  to  0.35  ampere  and  a  potential  of  7  volts.  The 
metal  was  completely  precipitated  at  the  end  of  7  hours, 
and  was  bright  and  adherent. 

LEAD. 
LITERATURE: 

Becquerel,  Compt.  rend.,  No.  26  (1854). 

Luckow,  Dingl.  Polyt.  Journ.,  p.  177  (1865). 

May,  Am.  Journ.  Science,  [3]  6,  255  (1873). 

Hampe,  Zeit.  f.  anal.  Chem.,  13,  183  (1874). 

Parodi  and  Mascazzini,  Ber.  deutsch.  chem.  Ges.,  10,  1098  (1877). 


192  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Parodi  and  Mascazzini,  Zeit.  f.  anal.  Chem.,  16,  469  (1877); 

ibid.,  18,  588  (1879). 
Riche",  Zeit.  f .  anal.  Chem.,  17,  219  (1878) ; 

Ann.  d.  Chim.  et  Phys.,  13,  508  (1878). 
Luckow,  Zeit.  f.  anal.  Chem.,  19,  215  (1880). 
Classen,  Zeit.  f.  anal.  Chem.,  21,  257  (1882). 
Schucht,  Zeit.  f.  anal.  Chem.,  21,  488  (1882). 
Riche",  Zeit.  f.  anal.  Chem.,  21,  117  (1882). 
Kiliani,  Berg-  u.  Huttenm.  Zeitung,  p.  253  (1883). 
Tenny,  A.  Chem.  Journ.,  5,  413  (1884). 
Smith,  Pr.  Am.  Phil.  Soc.,  24,  428  (1886). 
Brand,  Zeit.  f.  anal.  Chem.,  28,  581  (1889). 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749  (1891). 
Riidorff,  Zeit.  f.  angew.  Chem.,  p.  198  (1892). 
Warwick,  Zeit.  f.  anorg.  Chem.,  i,  258  (1892). 
Medicus,  Ber.  deutsch.  chem.  Ges.,  25,  2490  (1892). 
Nissenson,  Zeit.  f.  angew.  Chem.,  p.  646  (1893). 
Nissenson  and  Riist,  Zeit.  f.  anal.  Chem.,  32,  424  (1893). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894) ; 

Ber.  deutsch.  chem.  Ges.,  27,  163  (1894). 
Kreichgauer,  Ber.  deutsch.  chem.  Ges.,  27,  315  (1894); 

Zeit.  f.  anorg.  Chem.,  9,  89  (1895). 

Nissenson  and  Neumann,  Chem.  Ztg.,  19,  177,  1143  (1895). 
Neumann,  Chem.  Ztg.,  p.  381  (1896). 
Hollard,  Bull.  Soc.  Chim.,  19,  911  (1898). 
Nissenson,  Chem.  Ztg.,  23,  868  (1900). 
Marie,  Chem.  Ztg.,  24,  314,  480  (1900). 
Linn,  Journ.  Am.  Chem.  Soc.,  24,  435  (1902). 


If  a  solution  of  a  lead  salt  containing  an  excess  of  ammo- 
nium oxalate  be  electrolysed  warm,  the  lead  separates  at 
the  negative  electrode,  adheres  firmly,  and  shows  its  char- 
acteristic metallic  properties;  but  it  oxidises  partially  on 
washing  with  water  and  alcohol,  so  that  the  results  are  always 
too  high.  The  precipitation  of  lead  as  amalgam  presents 
some  difficulties,  inasmuch  as  some  lead  peroxide  separates 
at  the  positive  electrode  and  must  be  dissolved.  According 
to  G.  Vortmann,  the  aqueous  solution  of  the  lead  salt,  con- 
taining sufficient  mercuric  chloride  to  produce  the  amalgam, 


LEAD.  193 

is  treated  with  3-5  g  sodium  acetate  and  a  few  cubic  centime- 
ters of  concentrated  potassium  nitrite  solution.  The  pre- 
cipitate produced  by  the  latter  reagent  (which  is  added  to 
prevent  the  formation  of  peroxide)  is  dissolved  in  acetic  acid, 
and  the  clear  yellow  solution  diluted  and  electrolysed.  If 
lead  peroxide  appears  on  the  positive  electrode  during  the 
reaction,  more  potassium  nitrite  is  added.  The  end  of  the 
reaction  is  determined  by  testing  with  ammonium  sulphide. 
As  lead  amalgam  oxidises  rather  readily  'when  moist,  it  is 
quickly  washed  with  water,  alcohol,  and  ether,  dried  by  the 
warmth  of  the  hand  and  by  blowing  upon  it,  and  finally  in  the 
desiccator. 

The  amalgam  may  also  be  separated  from  an  aqueous  solu- 
tion acidified  with  nitric  acid.  However,  as  free  nitric  acid 
favors  the  formation  of  lead  peroxide,  more  frequent  addi- 
tion of  potassium  nitrite  is  necessary,  and  complete  precipi- 
tation is  thereby  seriously  hindered. 

In  a  solution  containing  free  nitric  acid,  lead  is  acted  on 
like  manganese;  it  is  oxidised,  and  separates  as  hydrated 
peroxide  at  the  positive  electrode.  If  there  is  no  other  metal 
in  the  solution,  it  must  contain  at  least  10  per  cent,  free  nitric 
acid,  according  to  Luckow;  in  the  presence  of  other  metals 
(mercury,  copper,  etc.),  the  oxidation  is  complete  even  in 
the  presence  of  little  nitric  acid. 

In  the  Munich  laboratory  experiments  have  been  con- 
ducted as  to  the  quantity  of  nitric  acid  (sp.  gr.  1.36),  and  have 
demonstrated  that  this  depends  on  the  temperature  and 
the  current-density  which  is  used.  The  current-density  de- 
pends in  turn  on  the  condition  of  the  surface  of  the  positive 
electrode.  If  this  is  very  smooth,  a  current  of  ND100  =  0.05 
is  sufficient,  otherwise  one  of  ND100  =  0.5  is  needed  to  pro- 
duce an  adherent  precipitate.  When  ND100  =  0.05  ampere, 
2  per  cent,  by  volume  of  nitric  acid  should  be  added  when  the 


194  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

solution  is  heated,  and  10  per  cent,  by  volume  at  ordinary 
temperatures.  When  ND100  =  0.5  the  volume-percentages 
are,  respectively,  7  and  20  for  heated  and  cool  solutions. 

Heating  the  solution  to  about  50°  materially  assists  the 
separation.  The  precipitate  may  be  washed  without  loss, 
after  the  current  is  interrupted. 

Chlorine  compounds  must  not  be  present  in  the  solution 
for  electrolysis. 

Even  when  the  stated  conditions  are  observed,  the  quan- 
tity of  lead  which  can  be  precipitated  as  peroxide  in  an  ad- 
herent form  is  relatively  small.*  The  rapid  separation  of 
large  quantities  of  lead  peroxide,  firmly  adherent  like  a  metal, 
may  only  be  carried  out  without  difficulty,  as  the  author's  re- 
searches have  shown,  when  the  inside  of  the  platinum  dish 
serving  as  anode  is  roughened  with  a  sand-blast,  f  By  the 
use  of  such  dishes  it  is  possible,  with  a  current  of  1.5  am- 
pere, to  precipitate  in  a  few  hours  as  much  as  4  g  of  lead  per- 
oxide on  100  sq.  cm  of  surface. 

For  conducting  the  determination  of  lead,  after  the  solu- 
tion of  the  lead  salt  has  been  accomplished,  20  cc  nitric  acid 
(sp.  gr.  1.35-1.38)  are  added,  the  solution  is  diluted  to  about 
100  cc,  warmed  to  60-65°,  and  electrolysed  with  a  current 
of  ND100  =  1.5-1.7  amperes.  If  the  warming  is  continued 
during  the  electrolysis,  the  precipitation  of  quantities  up  to 
1.5  g  lead  peroxide  is  completed  in  about  3  hours;  with  larger 
quantities  in  about  4-5  hours.  Complete  precipitation  is 

*  From  experience  in  the  Aachen  laboratory,  the  greatest  possible 
quantity  is  0.15  g  PbO2  per  100  sq.  cm  surface,  while  according  to  the 
statements  of  Dr.  Cohen  (Chem.  Ztg.,  1893,  No.  98)  as  much  as  0.3  g  can 
be  precipitated. 

t  The  platinum  refinery  of  G.  Siebert  in  Hanau  has  faultlessly  carried 
out  the  roughening  in  the  desired  manner  at  the  request  of  the  author. 
Such  roughened  dishes  are  of  course  applicable  to  all  other  electrolytic 
determinations. 


LEAD.  195 

insured  by  adding  about  20  cc  of  water  and  observing 
whether  the  freshly  wetted  surface  of  the  electrode  becomes 
darker.  In  case  no  blackening  is  observed  at  the  end  of  10-15 
minutes,  the  current  is  stopped  and  the  precipitate  is  washed 
with  water  and  alcohol  and  dried  at  180-190°.  The  residue 
is  anhydrous  peroxide. 

CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  acetate  or  nitrate. 

Substance  added :  at  least  10%  (by  volume)  of  nitric  acid 
(sp.  gr.  1.35). 

Total  volume  of  solution :  120  cc. 

Temperature:  60°  to  70°. 

Current-density  at  anode:   ND100=1.0  to  2.0  amperes. 

Potential-difference  between  electrodes:   2.3  to  2.7  volts. 

Time  required :  3  to  5  hours. 

Where  the  quantity  of  peroxide  to  be  separated  is  at  all 
considerable,  roughened  dishes  must  be  used. 

EXPERIMENT. 

Used  lead  nitrate  (Pb  =  72.21%)  dissolved  in  100  cc 
water,  with  the  addition  of  20  cc  nitric  acid  (sp.  gr.  1.35-1.38). 

CUI2S^!ity>  Electr^otentia1'    Temp.                   Time.                    Found. 

1.55-1.45  2.43-2.4           60-65°  1  hr.  5  m.  72.20% 

1.6-1.58  2.48-2.43         60-65°  1    "10"  72.19    " 

1.6-1.65  2.41-2.36        60-65°  1   "   5  "  72.20    " 

Under  these  conditions  lead  can  be  separated  from  zinc, 
iron,  cobalt,  nickel,  aluminium,  and  magnesium,  which 
remain  in  solution,  and  also  from  copper,  antimony,  gold, 
mercury,  and  cadmium,  which  are  wholly  or  partially  pre- 
cipitated on  the  cathode.  Silver  and  bismuth,  which  sepa- 
rate partly  as  metals  on  the  cathode  and  partly  as  oxides 
on  the  anode,  interfere  with  the  determination;  but  if  20% 
of  nitric  acid  is  contained  in  the  solution  these  latter  ele- 


196  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

ments  do  not  interfere  with  the  results  unless  present  in 
considerable  quantity. 

The  presence  of  arsenic  is  very  objectionable,  and  when 
enough  (0.05  g)  is  present  no  lead  is  deposited  as  peroxide, 
but  instead  it  is  precipitated  as  metal  mixed  with  arsenic  on 
the  cathode.  If  the  electrolysis  is  continued  for  some  time 
the  arsenic  is  gradually  eliminated  from  the  solution  as 
arseniuretted  hydrogen  and  the  precipitated  metals  again 
pass  into  the  solution.  If  the  electrolysis  is  conducted  for 
a  sufficiently  long  period,  all  of  the  lead  will  ultimately 
be  precipitated  as  peroxide  on  the  anode.  The  effect  of 
selenium  is  similar  to  that  of  arsenic.  In  conducting  separa- 
tions, the  solution  should  always  contain  20%  of  nitric  acid. 

In  the  analysis  of  substances  containing  lead  and  sulphur, 
especially  when,  as  is  often  necessary,  these  are  decomposed 
.with  nitric  acid,  a  precipitate  of  insoluble  lead  sulphate  is 
frequently  obtained.  Lead  sulphate  can  be  brought  readily 
into  solution  under  conditions  suitable  for  the  electrolytic 
determination  of  the  lead  by  the  following  treatment:  A 
slight  excess  of  ammonia  is  added  and  the  mixture  is  warmed 
for  a  short  time;  this  converts  the  lead  sulphate  into  porous 
lead  hydroxide.  With  constant  stirring,  the  mixture  is 
now  poured,  little  by  little,  into  a  platinum  dish  containing 
about  20  cc  of  warm  nitric  acid,  and  the  lead  sulphate  which 
reappears  either  dissolves  immediately  or,  if  the  quantity  is 
large,  the  greater  part  goes  at  once  into  solution  and  the  re- 
mainder dissolves  slowly  on  warming  the  solution  for  a  short 
time.  The  vessel  in  which  the  decomposition  of  the  lead 
sulphate  has  been  conducted  is  first  washed  with  a  little 
nitric  acid  and  then  with  water,  the  washings  being  added 
to  the  solution  in  the  platinum  dish. 


THALLIUM. 


197 


THALLIUM. 
LITERATURE : 

Schucht,  Zeit.  f.  anal.  Chem.,  22,  241,  490  (1883). 
Neumann,  Ber.  deutsch.  chem.  Ges.,  21,  356  (1888). 

This  metal  can  be  completely  precipitated  from  an  am- 
monium oxalate  solution. 

The  properties  of  thallium,  however,  are  similar  to  those 
of  lead;  its  determination,  therefore,  requires  special  con- 
sideration. 

G.  Neumann,  in  connection  with  a  research  on  certain 
double  salts  of  thallium  in  the  Aachen  laboratory,  has  also 
investigated  the  quantitative 
determination  of  the  metal. 
As  his  method  is  of  value  in 
the  investigation  of  thallium 
compounds,  it  is  here  described. 
The  process  is  based  on  pre- 
cipitation of  the  thallium  as 
metal,  and  determination  of  the 
volume  of  hydrogen  set  free  by 
its  solution  in  hydrochloric  acid. 

The  apparatus  shown  in  Fig. 
92  is  used  for  the  process.  A; 
is*  a  flask  of  about  100  cc 
capacity,  containing  platinum- 
foil  electrodes  of  9  sq.  cm 
surface,  terminating  in  contact- 
wires  fused  into  the  glass.  The 
thallium  salt  and  about  5  g 
ammonium  oxalate  are  dis- 
solved in  this  flask  and  electro- 
lysed, after  dilution,  with  a  current  of  0.1  ampere.  The 
completion  of  the  reaction  is  ascertained  by  testing  with  am- 


FIG.  92. 


198 


QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS. 


monium  sulphide.  As  the  ammonium  oxalate  is  converted 
into  carbonate  by  the  current,  and  the  measuring-tube  would 
be  insufficient  to  contain  the  liberated  carbon  dioxide, 
the  solution  remaining  in  the  flask  is  removed  after  the  reac- 
tion. This  may  readily  be  done  by  the  use  of  two  siphons. 
Neumann's  automatic  arrangement  for  this  purpose  is  shown 
in  Fig.  93;  it  is  very  convenient  where  many  determinations 
are  to  be  performed,  and  its  operation  is  easily  seen  from 
the  figure.  The  washing  is  conducted  without  interrupting 
the  current.  To  remove  the  gas  bubbles  clinging  to  the 


FIG.  93. 

electrode  it  is  desirable  to  heat  the  flask  a  short  time  after 
the  washing  is  complete.  The  flask  is  then  connected  to 
the  measuring-tube,  the  thallium  dissolved,  and  the  hydrogen 
collected  and  measured  in  the  usual  way. 


SILVER.  199 


SILVER. 

LITERATURE : 

Luckow,  Dingl.  Polyt.  Journ.,  178,  43  (1880); 

Zeit.  f.  anal.  Chem.,  19,  15  (1880). 

Fresenius  and  Bergmann,  Zeit.  f.  anal.  Chem.,  19,  342  (1880). 
Schucht,  Zeit.  f.  anal.  Chem.,  19,  316  (1880). 
Krutwig,  Ber.  deutsch.  chem.  Ges.,  15,  1267  (1882). 
Kinnicutt,  Am.  Chem.  Journ.,  4,  22  (1882). 
Schucht,  Zeit.  f.  anal.  Chem.,  22,  417  (1883). 
Smith,  Journ.  Anal.  Chem.,  3,  254,  385  (1889). 
Frankel,  Am.  Chem.  Journ.,  n,  264,  352  (1890). 
Smith,  Am.  Chem.  Journ.,  12,  335  (1891). 
Riidorff,  Zeit.  f.  angew.  Chem.,  p.  5  (1892). 
Smith  and  Spencer,  Elektrochem.  Zeit.,  i,  542  (1894). 
Eisenberg,  Dissertation,  Heidelberg  (1895). 
Hardin,  Journ.  Am.  Chem.  Soc.,  18,  990  (1896). 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 
Fulweiler  and  Smith,  Journ.  Am.  Chem.  Soc.,  23,  582  (1901). 
Gooch  and  Medway,  Am.  Journ.  of  Science,  April,  1903. 

Of  the  methods  proposed  for  the  determination  of  silver, 
the  one  suggested  by  Luckow  (separation  of  the  silver  from 
the  potassium  double  cyanide)  is  probably  the  most  suitable. 
If  insoluble  silver  compounds  (silver  chloride,  silver  oxalate) 
are  to  be  analysed,  they  are  dissolved  in  potassium  cyanide 
solution.  For  conducting  the  method,  3  g  potassium  cyanide 
are  added  to  the  solution,  which  is  then  diluted  to  100-120  cc. 
Eisenberg,  who  tested  the  method  in  the  Aachen  laboratory, 
was  convinced  that  its  successful  performance,  as  well  as  the 
metallic  condition  of  the  precipitated  silver,  depends  upon  the 
purity  of  the  potassium  cyanide  used.  Even  the  so-called 
' '  purissimum  "  potassium  cyanide  of  commerce  is  unsuited. 
It  is  therefore  desirable  to  prepare  pure  potassium  cyanide  by 
passing  hydrocyanic,  acid  gas  into  an  alcoholic  solution  of 
potassium  hydroxide. 


200  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 


CONDITIONS   FOR   ANALYSIS. 

Metal  present  as  nitrate  or  sulphate. 

Substance  added:  3  g  of  pure  potassium  cyanide. 

Total  volume  of  solution :  100  to  120  cc. 

Temperature:  20°  to  30°. 

Current-density  at  cathode:   ND100  =  0.2  to  0.5  ampere. 

Potential-difference :    3.7  to  4.8  volts. 

Time  required:   1J  to  5  hours. 

For  this  determination  roughened  dishes  give  best  results. 

J.  Krutwig  treats  the  solution  of  the  silver  salt  with 
ammonia  in  slight  excess,  adds  ammonium  sulphate,  and 
electrolyses. 

In  the  Munich  laboratory  the  following  conditions  have 
been  determined  for  the  preceding  process.  The  solution, 
which  must  not  contain  more  than  0.5  g  silver,  is  treated 
with  20  per  cent,  by  volume  of  ammonia  (sp.  gr.  0.96)  and  5% 
ammonium  sulphate  solution  (1 : 10),  warmed,  and  elec- 
trolysed with  a  current  of  ND100  =  0.02-0.05  ampere.  After 
the  current  is  stopped  the  precipitate  must  be  very  thor- 
oughly washed  to  completely  remove  the  ammonium  sulphate. 

Fresenius  and  Bergmann  have  found  that  silver  can  also 
be  precipitated  in  a  dense  form  from  a  solution  containing 
nitric  acid:  20  cc  of  nitric  acid  (sp.  gr.  1.2)  are  added  to  the 
silver  solution,  which  is  then  diluted  with  water  to  about 
200  cc  and  electrolysed. 

According  to  results  in  the  Munich  laboratory,  it  is  desir- 
able to  add  to  the  solution,  which  may  contain  as  much  as  0.4 
g  silver,  3  per  cent,  by  volume  of  nitric  acid,  sp.  gr.  1.36,  and 
to  electrolyse  the  heated  solution  with  a  current  of  ND100= 
0.04-0.05  ampere.  The  silver  must  be  carefully  washed 
without  interrupting  the  current,  to  prevent  loss.  An  insuf- 


SILVER.  201 

ficient  quantity  of  nitric  acid  may  lead  to  the  formation  of 
peroxide. 

Silver  can  be  obtained  as  a  white  strongly  adhering 
deposit  on  roughened  dishes  by  electrolysing  a  solution 
containing  1  to  2  cc  of  nitric  acid  (sp.  gr.  1.4)  and  5  cc  of 
alcohol,  if  the  potential-difference  between  the  electrodes 
is  carefully  regulated  so  as  to  be  within  the  limits  1.35-1.38 
volt.  The  time  required  for  precipitation  is  from  6  to  8 
hours,  and  depends  but  little  on  the  amount  of  silver  contained 
in  the  solution.  A  quantity  of  from  0.1  to  0.5  gram  of  silver 
is  convenient,  but  the  quantity  may  be  as  great  as  2  grams. 
The  chief  factor  of  importance  is  the  potential-difference, 
which  must  be  kept  within  the  limits  specified,  since  an  in- 
crease to  even  1.4  volt  causes  the  silver  to  separate  in  a 
spongy  form,  which  is  useless  for  quantitative  determination.* 

Kollock  describes  an  experiment  under  the  following 
conditions:  0.1270  g  of  silver,  present  as  nitrate,  0.5  to  1  g 
of  potassium  cyanide,  temperature  65°,  total  volume  100  cc, 
ND100  =  0.04  to  0.07,  potential-difference  2.5  to  3.2  volts, 
time  required  3  to  5  hours. 

A  further  experiment  is  described  by  Fulweiler  and 
Smith  as  follows:  0.2133  g  silver  present,  2  g  potassium 
cyanide  added,  total  volume  125  cc,  temperature  60-65°, 
ND100  =  0.03-0.04,  potential-difference  2.5-2.7  volts,  time 
required  for  complete  precipitation  of  the  silver  3-4  hours. 

For  the  determination  of  silver  by  precipitation  on  a 
rapidly  rotating  cathode  (p.  124),  Gooch  and  Medway  used 
a  solution  prepared  by  adding  to  50  cc  of  a  silver  nitrate 
solution  enough  potassium  cyanide  to  dissolve  the  silver 
cyanide  at  first  formed.  Three  cubic  centimeters  of  dilute 
sulphuric  acid  were  then  run  in,  and  enough  ammonia  was 

*  Classen,  Ausgewahlte  Methoden,  p.  3. 


202  QUANTITATIVE    ANALYSIS    BY  ELECTROLYSIS. 

added  to  make  the  solution  strongly  alkaline.  The  weight 
of  silver  taken  varied  from  0.0968  to  0.1898  g,  and  the 
electrolysis  was  conducted  with  a  current  of  1.8-3  amperes 
(equivalent  to  ND100  =  6-10  amperes).  The  time  required 
for  the  complete  precipitation  of  the  silver  was  from  8  to  15 
minutes. 


MERCURY. 

LITERATURE : 

Clarke,  Am.  Journ.  Science,  16,  400  (1878) ; 

Ber.  deutsch.  chem.  Ges.,  n,  1140  (1878). 
Luckow,  Zeit.  f.  anal.  Chem.,  19,  14  (1880). 
Classen  and  Ludwig,  Ber.  deutsch.  chem.  Ges.,  19,  323  (1886). 
Hoskinson,  Am.  Chem.  Journ.,  8,  209  (1886). 
Smith  and  Kneer,  Am.  Chem.  Journ.,  8,  206  (1886). 
Smith  and  Frankel,  Am.  Chem.  Journ.,  n,  264  (1889). 
Smith,  Journ.  Anal.  Chem.,  5,  202  (1891). 
Smith  and  Cauley,  Am.  Chem.  Journ.,  12,  428  (1891); 
Journ.  Anal.  Chem.,  5,  489  (1891). 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2749  (1891). 
Frankel,  Journ.  Franklin  Inst.,  131,  144  (1891). 
Brand,  Zeit.  f.  angew.  Chem.,  p.  202  (1891). 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  110  (1891). 
Riidorff,  Zeit.  f,  angew.  Chem.,  p.  5  (1892). 
Smith  and  Moyer,  Journ.  Anal.  Chem.,  7,  252  (1893^. 
Smith  and  Wallace,  Journ.  Anal.  Chem.,  7,  189  (1893). 
Schmucker,  Journ.  Am.  Chem.  Soc.,  15,  204  (1893). 
Frangois,  Journ.  Pharm.  et  Chim.,  [5]  30,  249  (1894). 
Eisenberg,  Dissertation,  Heidelberg  (1895). 
Rising  and  Lehner,  Berg-u.  Huttenm.  Ztg.,  55,  175  (1896). 
Wallace  and  Smith,  Journ.  Am.  Chem.  Soc.,  18,  169  (1896). 
Hardin,  Journ.  Am.  Chem.  Soc.,  18,  990  (1896). 
Schmucker,  Zeit.  f.  anorg.  Chem.,  5,  206  (1896).  ,  V  . 

Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899), 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 
Bindschedler,  Zeit.  f.  Elektrochem.,  8,  329  (1902). 
Glaser,  Zeit.  f.  Elektrochem.,  9,  11  (1903).' 


MERCURY. 


203 


The  metal  can  be  readily  separated  from  solutions  of  the 
mercuric  salts  to  which  4-5  g  ammonium  oxalate  have  been 
added  (method  of  the  author).  If  the  mercury  is  present  as 
chloride  in  the  solution,  the  electrolysis  is  continued  until 
mercurous  chloride  disappears  from  the  positive  electrode. 

CONDITIONS   FOR    ANALYSIS. 

Substance  added :  4  to  5  g  ammonium  oxalate. 

Total  volume  of  solution:  120  to  150  cc. 

Temperature:  16°  to  40°. 

Current-density  at  cathode:  ND100  =  1.0  ampere. 

Potential-difference:  5.5  volts. 

Time  required:  2  hours. 

Roughened  .dishes  are  preferable  to  polished,  on  account 
of  the  more  uniform  distribution  and  firmer  adherence  of  the 
mercury  to  the  cathode.  On  polished  dishes  the  mercury 
separates  in  the  form  of  small  globules. 


EXPERIMENT. 


Subst. 
Used. 
HgCla. 
g. 

Current- 
density, 
Amperes. 
ND100 

Electrode 
Potential, 
Volts. 

Temp. 

Time, 
hrs.  m. 

Found.       Remark. 

0.4068 

0 

2-0 

15 

2 

.6  -3 

.35 

30-23° 

5 

15 

73.74^ 

0.4073 

1 

02-0 

93 

4 

.05-4 

.75 

29-37° 

1 

30 

73.63 

0.4076 

1 

08-0.92 

4 

.42-4 

.88 

25-40° 

2 

5 

73.77 

Roughened 

0.4080 

1 

15-1 

.09 

4 

.97-5 

.05 

18-40.5° 

2 

5 

73.87 

Dish. 

0.4080 

1 

12-0 

93 

4 

.95-4 

.85 

18-38° 

2 

5 

73.84 

0.4080 

1 

52-0 

48 

3 

.65-4 

.65 

16-27° 

3 

55 

73.67- 

0.4070 

0 

2  -0 

23 

2 

.89-3 

.75 

28-24° 

5 

15 

73.80] 

0.4073 

1 

06-0 

95 

4 

.45-5 

.00 

30-39.5° 

1 

30 

73.29 

0.4076 

1 

16-1 

09 

5 

.32-5 

.53 

23-40° 

2 

5 

73.93f    p^ed 

0.4080 

1 

20-0 

99 

4 

.70-4 

.90 

18-43° 

3 

— 

73.55  1 

0.4075 

1 

.51-0 

48 

3 

.87-4 

.50 

16-30° 

3 

55 

73.85J 

Theory  73. 85%  Hg. 


204  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Mercury  may  also  be  quantitatively  precipitated  from  a 
solution  containing  nitric,  sulphuric,  or  hydrochloric  acid. 
If  no  other  metal  than  mercury  is  present,  1-2  per  cent,  by 
volume  of  nitric  acid  is  sufficient;  while  in  the  presence  of 
other  metals,  which  are  not  precipitated  from  solutions  con- 
taining free  acid,  5  per  cent,  by  volume  is  required.  In  the 
latter  case  a  current-density  not  greater  than  0.5  ampere  is 
employed;  in  the  former  the  current-density  may  be  raised 
to  1  ampere.  Time,  about  2  hours. 

The  precipitation  of  mercury  from  a  nitric  acid  solution 
is  described  by  Kollock  as  follows:  0.1403  g  of  mercury  was 
dissolved  in  3  cc  of  concentrated  nitric  acid,  the  solution 
was  diluted  to  125  cc  and  electrolysed,  at  70°,  with  a  current 
of  ND100  =  0.06  ampere  and  a  potential-difference  of  2  volts. 
The  mercury  was  completely  precipitated  in  2  hours. 

The  same  author  describes  the  precipitation  of  mercury 
from  a  sulphuric  acid  solution  under  the  following  conditions: 
To  a  solution  containing  mercuric  chloride  equivalent  to 
0.1403  g  of  Hg,  1  cc  of  concentrated  sulphuric  acid  was 
added,  and  the  solution,  after  dilution  to  125  cc,  was  elec- 
trolysed at  65°  with  a  current  of  ND100  =  0.4-0.6  ampere 
and  a  potential-difference  of  3.5  volts.  The  precipitation 
of  the  mercury  was  complete  in  1  hour. 

If  hydrochloric  acid  is  used,  only  a  few  drops  are  added, 
since  larger  quantities  have  a  detrimental  action  on  the  sepa- 
ration of  the  metal.  Large  quantities  of  chlorides  have  an 
action  similar  to  that  of  large  quantities  of  hydrochloric  acid. 

Insoluble  mercury  compounds  may  be  easily  electrolysed 
by  suspending  them  in  water  slightly  acidified  with  hydro- 
chloric acid,  or  in  a  dilute  solution  of  sodium  chloride  (about 
10  per  cent.).  This  process,  originated  by  the  author,  is 
used  at  Almaden  for  determining  the  amount  of  mercury 
contained  in  cinnabar. 


MERCURY.  205 

Francois  has  shown  that  in  a  similar  manner  the  iodide, 
iodate,  chloride,  and  bromide  can  be  electrolysed  in  the  solid 
state.  0.5  g  of  the  solid  iodide  was  introduced  into  a  plati- 
num crucible,  which  served  as  cathode,  and  20  cc  of  a  solu- 
tion of  20  g  of  ammonium  nitrate  in  100  cc  ammonia  was 
added.  The  anode  was  a  platinum  wire  1  mm  in  diameter, 
which  dipped  for  a  distance  of  only  1  mm  into  the  liquid. 
The  contents  of  the  crucible  were  stirred  from  time  to  time 
with  a  glass  rod.  The  washing  was  conducted  first  with 
ammonia  and  then  with  water,  the  liquids  being  allowed  to 
stand  in  contact  with  the  precipitated  mercury  for  some 
time.  Finally  the  cathode  was  washed  with  alcohol  and 
ether  and  dried  at  a  low  temperature.  For  exact  results 
the  anode  should  be  weighed  before  and  after  the  electroly- 
sis to  determine  any  loss  of  platinum  from  this.* 

Edgar  F.  Smith  has  described  the  determination  of  mer- 
cury by  the  electrolysis  of  a  solution  containing  potassium 
cyanide.  The  solution  of  the  mercuric  salt,  which  may 
contain  about  0.2  g  mercury,  is  treated  with  0.25-2  g  of 
potassium  cyanide,  diluted  with  water  to  175  cc,  and  sub- 
mitted to  electrolysis. 

Heidenreich, '  working  in  the  Aachen  laboratory,  has 
determined  the  following  conditions  for  this  method. 

CONDITIONS  FOR  ANALYSIS. 

Metal  present  as  mercuric  salt. 

Substance  added :  2  to  3  g  potassium  cyanide. 

Total  volume  of  solution :  175  cc. 

Temperature :  that  of  room. 

Current-density:  ND100  =  0.03  to  0.08  ampere,     dt 

Potential-difference:  1.65  to  1.75  volt.  ,   ^ 

Time:  about  5  hours. 

*  Classen,  Ausgewahlte  .Methoden,  p.  50. 


206          QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

The  metal  reduced  by  this  method  must  be  washed  with 
water  only,  and  not  with  alcohol,  since  when  the  latter  is 
used,  small  quantities  of  mercury  will  become  loosened  and 
will  be  carried  away. 

The  following  experiment  has  been  described  by  Kollock : 
Mercuric  chloride  equivalent  to  0.1439  g  mercury,  0.5  g  po- 
tassium cyanide,  total  volume  100  cc,  temperature  65°, 
current-density  ND100  =  0.02-0.07  ampere,  potential-differ- 
ence 1.6-3.2  volts,  time  required  3-6  hours.  The  mercury 
was  completely  precipitated. 

Smith  and  Wallace  describe  the  precipitation  of  mercury 
from  a  sodium  sulphide  solution  under  the  following  condi- 
tions: To  a  solution  containing  0.1913  gram  of  mercury  as 
chloride  there  was  added  20  cc  of  a  sodium  sulphide  solution 
(sp.  gr.  1.22).  The  total  volume  of  the  solution  was  125  cc. 
The  electrolysis  was  carried  out  at  a  temperature  of  65°  with 
a  current  of  ND100  =  0.13  ampere.  The  mercury  was  com- 
pletely precipitated  in  3  hours. 

GOLD. 

LITERATURE ! 

Persoz,  Annal.  Chim.  Pharm.,  65,  164  (1848). 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  14  (1880). 

Bruhnatelli,  Phil.  Mag.,  21,  187  (1886). 

Smith  and  Muhr,  Ber.  deutsch.  chem.  Ges.,  23,  2175  (1890). 

Smith,  Journ.  Anal.  Chem.,  5,  2Q4  (1891). 

Frankel,  Journ.  Franklin  Inst.,  1891. 

Smith  and  Wallace,  Ber.  deutsch.  chem.  Ges.,  25,  779  (1892). 

Smith,  Am.  Chem.  Journ.,  13,  206  (1892). 

Smith  and  Muhr,  Am.  Chem.  Journ.,  13,  417  (1892). 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  695,   (1892). 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Gold  may  be  separated  in  a  compact  form  from  solutions 
of  gold  salts  containing  potassium  cyanide.  To  form  the 


GOLD.  207 

• 

double  cyanide,  about  3  g  of  potassium  cyanide  are 
added.  The  solution  is  then  electrolysed  at  ordinary  tem- 
peratures or  at  temperatures  between  50°  and  60°.  Since 
it  is  generally  supposed  that  the  gold  can  be  removed 
from  the  platinum  dish  with  aqua  regia  only  (an  opera- 
tion by  which  the  platinum  is  also  dissolved),  platinum 
dishes  coated  with  a  thin  deposit  of  silver  have  previously 
been  used  for  this  determination.  According  to  a  private 
communication  from  D.  W.  Dupre,  of  Stassfurt,  the  gold  may 
be  readily  removed  from  the  platinum  dishes  by  warming 
with  a  solution  of  chromic  anhydride  in  saturated  sodium 
chloride  solution.  The  author  can  confirm  this  statement; 
in  this  operation  gold  only,  and  no  platinum,  goes  into  solu- 
tion. According  to  Smith,  the  deposited  gold  can  be  removed 
from  the  cathode  by  allowing  it  to  stand  in  contact  with 
dilute  potassium  cyanide  solution,  the  electrode  being  con- 
nected as  anode  with  a  source  of  feeble  current. 

Since  the  conditions  for  the  separation  of  gold  from 
double  cyanides  had  not  been  previously  determined,  they 
were  ascertained  by  Dr.  v.  Wirkner  at  the  suggestion  of  the 
author. 

CONDITIONS   FOR   ANALYSIS. 

Substance  added :  3  g  potassium  cyanide. 

Total  volume  of  solution :  120  cc. 

Temperature:  50°  to  60°;  the  electrolysis  can  also  be 
conducted  at  room  temperature,  but  this  is  undesirable, 
since  a  brownish  decomposition  product  of  potassium 
cyanide  often  separates. 

Current-density  at  cathode:  ND100  =  0.3  to  0.8  ampere. 

Potential-difference:  2.7  to  4  volts. 

Time  required:  for  warm  solutions  1J  hour,  for  cold 
solutions  from  4  to  5  hours. 


208  QUANTITATIVE    ANALYSIS   BY    ELECTROLYSIS. 


EXPERIMENT. 

A  solution  of  chloride  of  gold  of  unknown  strength  was 
used.  The  electrolyses  were  carried  out  in  roughened  plat- 
inum-iridium  dishes  without  a  coating  of  silver.  Used  3  g 
potassium  cyanide,  120  cc  liquid. 


Taken,  cc. 
Gold  Chlo- 
ride Sol. 

Current- 
density, 
Amperes. 

Electrode 
Potential, 
Volts. 

Temper- 
ature. 

Time, 
hr.   m. 

Found, 
g- 

15 

0.3 

3.5-3.9 

20-27° 

5  — 

0.0545 

15 

0.35 

3.9-4.0 

22-28° 

14-  (overnight) 

0.0548 

30 

0.37 

3.6-3.9 

20-28° 

4  15 

0.1099 

15 

0.38 

2.7-3.8 

52-55° 

1   30 

0.0544 

15 

0.38 

2.7-3.4 

53-54° 

1  20 

0.0546 

15 

0.39 

2.7-3.8 

52-56° 

1   30 

0.0545 

15 

0.85 

4.0-4.1 

52-56° 

1   30 

0.0544 

Smith  states  *  that  gold  can  be  satisfactorily  precipitated 
from  a  solution  containing  sodium  sulphide  (sp.  gr.  1.18)  by 
a  current  of  0.1-0.2  ampere  when  the  total  volume  is  about 
125  cc. 

ANTIMONY. 

LITERATURE  I 

Bottcher,  Journ.  f.  prak.  Chem.,  73,  484  (1858). 

Gore,  Chem.  Gazette,  16,  59  (1858). 

Wrightson,  Zeit.  f.  anal.  Chem.,  15,  300  (1876). 

Parodi  and  Mascazzini,  ibid.,  18,  588  (1879). 

Luckow,  ibid.,  19,  13  (1880). 

Chittenden  and  Blake,  Trans.  Conn.  Acad.  Science,  7,  276  (1880). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622   (1881); 

ibid.,  17,  2467  (1884). 

Classen  and  Ludwig,  Ber.  deutsch.  chem.  Ges.,  18,  1104  (1885). 
Lecrenier,  Chemiker  Zeitung,  13,  1219  (1888). 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2762  (1891). 
Sanderson,  Ber.  deutsch.  chem.  Ges.  (Ref.)  p.  340  (1891). 
Smith  and  Muhr,  Journ.  Anal.  Chem.,  5,  448  (1891); 
Journ.  Anal.  Chem.,  7,  i89  (1893). 

*  Electro-Chemical  Analysis  (1902),  p.  111. 


ANTIMONY.  209 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  199  (1892). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Hollard,  Eel.  electr.,  26,  165  (1900). 
Ost  and  Klapproth,  Zeit.  f .  angew.  Chem.,  p.  827  (1900) ; 
Zeit.  f.  Electrochem.,  7,  376  (1900). 


Antimony  is  precipitated  from  a  hydrochloric  acid  solution, 
but  not  in  an  adherent  form.  If  potassium  oxalate  is  added 
to  the  solution  of  the  trichloride,  antimony  is  easily  reduced, 
but  adheres  even  less  firmly  than  in  the  former  case.  An 
adherent  metallic  deposit  can  be  obtained  by  adding  potas- 
sium tartrate,  but  the  separation  is  then  too  slow. 

The  precipitation  of  antimony  from  the  solutions  of  its 
sulpho-salts  is  complete  and  satisfactory.  If  ammonium 
sulphide  is  used  to  produce  a  double  salt,  it  must  contain 
neither  free  ammonia  nor  polysulphides.  Ammonium  hy- 
drosulphide,  therefore,  is  convenient  for  the  determination; 
it  should  be  kept  in  small  tightly  corked  bottles. 

When  a  solution  of  antimony  containing  ammonium  sul- 
phide is  electrolysed,  there  is  formed  over  the  metal  a  coating 
of  sulphur  which  cannot  be  washed  off  with  water.  When 
the  metal  is  washed  afterward  with  alcohol,  the  thin  coating 
of  sulphur  can  be  removed  by  rubbing  with  the  finger  or  a 
handkerchief  moistened  with  alcohol,  without  danger  of  loss. 

The  use  of  ammonium  sulphide  has  the  disadvantage  that, 
when  several  determinations  are  made  together,  the  odor 
becomes  unbearable.  For  this  reason  the  author  has  made 
a  series  of  experiments  with  potassium  and  sodium  monosul- 
phide  and  hydrosulphide,  the  results  of  which  show  that  the 
precipitation  of  antimony  from  double  salts  with  these  com- 
pounds proceeds  satisfactorily.  As  sodium  sulphide  (Na2S) 
is  one  of  the  salts  named  which  is  most  desirable  for  facil- 
itating the  separation  of  antimony  from  tin  and  arsenic,  the 


210  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

following  particulars  relate  exclusively  to  the  use  of  this  salt  * 
for  the  determination  of  antimony. 

.  The  following  equations  probably  represent  the  reactions 
which  take  place  in  the  electrolysis  of  the  antimony  sulpho- 
salt. 
At  the  cathode : 

Sb2S3+  3Na2S  +  6H  -  2Sb  +  GNaHS. 

At  the  anode : 

6NaHS+30  =  3Na2S2+3H20. 

The  reduction  of  antimony  from  the  prepared  sulpho- 
salts  can  be  carried  out  as  well  at  ordinary  as  at  higher  tem- 
peratures. In  the  first  case  the  determination  requires  17-18 
hours,  in  the  latter  about  2  hours.  If  the  separation  is  con- 
ducted in  polished  dishes,  only  relatively  small  quantities 
of  the  metal  can  be  made  to  adhere  firmly  to  the  dish,  and  the 
employment  of  weak  currents  is  necessary.  In  recent  ex- 
periments roughened  dishes  have  been  used,  and  the  pre- 
cipitation has  been  conducted  from  hot  solutions  and  with 
stronger  currents. 

To  carry  out  the  analysis,  80-100  cc  of  a  solution  of 
sodium  monosulphide  (sp.  gr.  about  1.14)  are  added  to  the 
antimony  solution,  which  is  diluted  with  water  to  120  cc  and 
electrolysed.  If  the  metal  is  precipitated  from  a  warm 
solution,  it  must  be  washed  without  interrupting  the  current. 
The  end  of  the  reaction  can  only  be  determined  with  certainty 
by  the  use  of  another  electrode  which  is  dipped  into  the 
liquid  and  brought  into  contact  with  the  dish,  i.e.,  the 
cathode,  f 

*  For  the  preparation  of  this  salt,  see  section  on  Reagents. 
t  See  page  130. 


ANTIMONY.  211 

The  dish  with  the  separated  antimony  is  treated  in  the 
usual  way  with  water  and  perfectly  pure  absolute  alcohol, 
dried  for  a  short  time  in  the  air-bath  at  80-90°,  and  weighed. 

CONDITIONS   FOR   ANALYSIS. 

Composition  and  volume  of  the  solution  as  described 
above : 

A. 

Temperature :  ordinary  room  temperature. 
Current-density  at  cathode:   ND100  =  0.3  to  0.35  ampere. 
Potential-difference:  1.0  to  1.8  volts. 
Time  required:  17i  hours. 

B. 

Temperature:  55°  to  70°. 

Current-density  at  cathode:  ND100  =  1.0  to  1.5  amperes. 

Potential-difference :  1  to  2  volts. 

Time  required:  2^  hours. 

The  method  of  determining  antimony  in  solutions  of  the 
polysulphides  of  the  alkalies  is  very  simple.  The  solution 
containing  polysulphides  is  treated  with  an  excess  of  ammo- 
niacal  hydrogen  peroxide  and  heated  till  it  becomes  color- 
less. If  a  great  excess  of  hydrogen  peroxide  is  used,  it  may 
happen  that  the  alkali  sulphide  is  entirely  decomposed  and 
antimony  sulphide  precipitated.  If  the  solution  is  entirely 
colorless,  or  if  a  precipitate  of  antimony  sulphide  has  al- 
ready appeared,  the  solution  is  cooled,  80  cc  of  a  solution  of 
sodium  monosulphide  are  added,  the  whole  is  diluted  with 
water  to  about  120-150  cc  and  .electrolysed  as  above  di- 
rected. 

According  to  Lecrenier,  the  presence  of  polysulpliides 
can  be  counteracted  by  adding  50  cc  to  75  cc  of  a  solution 


212  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

of  sodium  sulphite  (20%)  and  heating  until  the  liquid  is 
colorless.  This  reaction  depends  upon  the  formation  of 
sodium  thiosulphate  from  the  sulphite,  sulphide,  and  sul- 
phur of  the  polysulphides. 

TIN. 
LITERATURE  I 

Luekow,  Zeit.  f.  anal.  Chem.,  19,  13  (1880). 

Gibbs,  Chem.  News,  42,  291  (1880). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  (1881). 

Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2467  (1886);  18,  1104  (1887). 

Bongartz  and  Classen,  ibid.,  21,  2900  (1888). 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  121  (1891). 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  196  (1892). 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 

Engels,  Zeit.  f.  Elektrochemie,  2,  418  (1895-96). 

Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1586  (1895). 

Campbell  and  Champion,  Journ.  Am.  Chem.  Soc.,  20,  687  (1898). 

Ost  and  Klapproth,  Zeit.  f.  angew.  Chem.,  p.  817  (1901). 

Tin  separates  completely  from  a  solution  containing  the 
ammonium  double  oxalate,  or  from  an  ammonium  sulphide 
solution.  Sodium  and  potassium  sulphides  cannot  be  used, 
as  tin  separates  only  partially  from  a  dilute  solution  of  the 
corresponding  sulpho-salt,  and  not  at  all  from  a  concentrated 
solution. 

If  tin  is  precipitated  from  the  ammonium  double  oxalate, 
a  separation  of  stannic  acid  readily  occurs,  especially  when 
much  tin  is  present,  which  must  be  redissolved  by  addition 
of  oxalic  acid.  The  reduction  of  tin  may  be  carried  out 
without  difficulty,  however,  if  acid  ammonium  oxalate  is  used 
instead  of  the  neutral  oxalate.  The  results  obtained  by  this 
process  are  so  accurate  that  the  author  has  found  it  adapted 
to  the  determination  of  the  atomic  weight  of  tin. 

The  solution  of  tin  is  treated  with  a  cold  saturated  solu- 
tion of  acid  ammonium  oxalate  in  the  proportion  of  20  cc  to 


TIN.  213 

0.1  g  tin.  The  solution  is  diluted  to  about  150  cc  and  elec- 
trolysed. The  tin  is  completely  precipitated  as  a  closely  ad- 
herent, shining,  silver-white  metal,  even  when  as  much  as 
6  g  is  present.  The  current  is  interrupted  and  the  metal 
washed  as  usual  with  water  and  alcohol,  and  dried  at  80-90°. 

CONDITIONS   FOR   ANALYSIS. 

Composition  and  volume  of  the  solution  as  described  above. 
Temperature :  ordinary  room  temperature. 
Current-density  at  cathode :  0.2  to  0.6  ampere. 
Potential-difference:  2.7  to  3.8  volts. 
Time  required :  8  to  10  hours. 

EXPERIMENT. 

Used  0.9-1  g  SnCl4.2NH4Cl  [32.10%  Sn],  120  cc  of  a  satu- 
rated solution  of  acid  ammonium  oxalate. 


Current-density, 
Amperes.  - 

Electrode 
Potential, 
Volts. 

Temp. 

Time. 

Found. 

0.2-0.3* 
0.3-0.6 

2.7-3.8 
2.8-3.8 

25° 

30-35° 

8hr.    5  m. 
9"    45  " 

32.06% 
32.00" 

If  larger  quantities  of  the  tin  salt  are  used,  it  is  necessary 
to  add  acid  ammonium  oxalate  from  time  to  time,  on  account 
of  the  decomposition  of  the  acid  ammonium  oxalate,  which 
causes  the  solution  to  react  alkaline.  According  to  recent 
investigations,  the  determination  of  tin  may  be  carried  out 
by  treating  the  solution  of  the  tin  salt  with  neutral  ammo- 
nium oxalate  to  form  the  double  salt,  acidifying  with  oxalic 
acid  and  electrolysing  warm. 

Heidenreich,  who  tested  this  method  in  the  Aachen  lab- 
oratory, found  that  the  determination  of  tin  can  be  com- 
pleted in  4-4J  hours.  4  g  ammonium  oxalate  to  every  0.3 
g  tin  present  are  added  to  the  solution,  which  is  then  acidi- 
fied with  9-10  g  oxalic  acid,  warmed  to  60-65°,  and  electro- 
*  Finally  increased  to  0 . 5  ampere. 


214  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

lysed  with  a  current  of  ND100  =  1-1.5  amperes.  The  pre- 
cipitate must  be  washed  without  interrupting  the  current. 

Instead  of  oxalic  acid,  acetic  acid  may  be  used;  it  pos- 
sesses, however,  no  advantages.  100  cc  of  a  saturated  solu- 
tion of  ammonium  oxalate  are  added  to  the  solution  of  the  tin 
salt,  which  is  then  acidified  with  25  cc  acetic  acid  (sp.  gr. 
1.0615;  about  50%).  The  metal  is  precipitated  in  the  form 
of  radiated  crystals,  in  contrast  to  the  precipitate  from  acid 
ammonium  oxalate  solutions.  Tin  adheres  better  to  rough- 
ened than  to  polished  dishes. 

The  following  experiments  were  conducted  by  the  acetic 
acid  method: 

p,          ,   j       -.  Electrode 

Ampere  Potential,  Temp.  Time.  Found. 

0.3  increased  to  0.5          3.2-3.8  .25°  6  hr.  15  m.          32.00% 

0.5       "          "1.0          3.6-4.2          25-30°          5  "  45  "  32.01" 

In  these  experiments  the  tin  in  the  polished  dishes  ap- 
peared brilliantly  crystalline,  and  in  the  roughened  dishes 
silver-white. 

Since  tin,  like  zinc,  is  dissolved  with  difficulty  from  the 
platinum  dishes  by  acids,  it  is  necessary  to  use  fused  acid 
potassium  sulphate  to  remove  it.*  It  is  therefore  best  to 
precipitate  the  tin  in  coppered  dishes  (see  Zinc). 

Engels  worked  out  the  following  method  in  the  Aachen 
laboratory:  The  tin  salt  is  dissolved  in  water  containing  a 
few  cubic  centimeters  of  oxalic  acid,  and  0.3-0.5  g  hydroxyl- 
amine,  2  g  ammonium  acetate,  and  2  g  tartaric  acid  are  added 
for  every  0.5-1.2  g  tin  salt  taken.  The  solution  is  then  diluted 
to  150  cc,  warmed  to  60-70°,  and  electrolysed  with  a  current 

*  The  removal  of  the  deposited  tin  from  the  electrode  can  usually  be 
effected  without  difficulty  by  warming  with  a  mixture  of  5  g  tartaric  acid, 
8  cc  water,  and  2  cc  of  concentrated  nitric  acid. 


TIN. 


215 


of  ND100  =  0.7  to  1.0  ampere  and  a  potential-difference  of 
4  to  5  volts.  Under  these  conditions  the  time  required 
for  complete  precipitation  of  the  tin  is  3  to  5  hours. 


EXPERIMENT. 


SnCl4. 
2NH4C1, 
g. 

Current- 
density, 
Amp. 

Electrode 
Potential, 
Volts. 

Temp. 

Time. 

Fc 
g- 

Per( 

0.9175 

1-0.8 

5.2-5.6 

70° 

3  hr.      0 

2970 

32 

37     32.37% 

0.9859 

1-0.8 

4.8-5.3 

63° 

3 

0 

3195 

32. 

40 

0.9050 

1-0.9 

5.0-5.6 

65° 

3 

0 

2931 

32. 

39 

1  .  1879 

0.5 

5.1-6.0 

45° 

6 

0 

3847 

32. 

38 

1.0026 

0.7 

3.4 

60° 

3 

0 

3238 

32. 

36 

0.9940 

0.7 

4.0 

60° 

3| 

0 

3219 

32. 

38 

1.0024 

0.8 

4.6 

60° 

3 

0 

3250 

32. 

42 

1.0022 

0.8 

4.2-4.4 

60° 

3 

0 

3252 

32. 

44 

In  the  solution  of  the  ammonium  sulpho-salt  tin  behaves 
like  antimony.  The  tin  solution  (if  necessary  after  neutral- 
isation with  ammonia)  is  treated  with  ammonium  sulphide 
free  from  ammonia  (no  more  is  added  than  is  needed  to  form 
the  sulpho-salt),  diluted  to  150-175  cc,  warmed  to  50-60°, 
and  electrolysed  with  a  current  of  1-2  amperes,  at  a  potential- 
difference  of  3.5-4  volts.  Under  these  conditions  0.3-0.4  gof 
tin  can  be  reduced  in  an  hour.  Sometimes  a  deposit  of  sul- 
phur adheres  so  strongly  to  the  tin  at  the  edge  of  the  dish 
that  it  cannot  be  washed  off  with  water;  it  may,  however, 
be  easily  removed,  after  washing  with  alcohol,  by  gentle 
rubbing  with  a  linen  cloth. 

In  gravimetric  analysis  tin  is  often  separated  from  other 
metals  by  sodium  sulphide  instead  of  ammonium  sulphide. 
In  order  to  determine  the  tin  electrolytically  in  such  cases, 
the  sodium  sulphide  must  be  converted  into  ammonium  sul- 
phide.* To  accomplish  this,  the  solution  is  treated  with 
about  23  g  pure  ammonium  sulphate  free  from  iron,  and 

*  Sodium  sulphide  cannot  be  replaced  by  potassium  sulphide  in  the 
separation  from  other  metals,  because  the  latter  produces  difficultly  soluble 
potassium  sulphate  when  ammonium  sulphide  is  formed. 


216  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

heated  very  carefully,  with  the  dish  covered,  till  the  hydro- 
gen sulphide  has  all  escaped;  the  solution  is  then  kept  in 
gentle  ebullition  for  about  fifteen  minutes.  Complete  con- 
version into  ammonium  sulphide  is  shown  by  the  greenish- 
yellow  color  of  the  solution.  If  the  heating  is  continued 
too  long,  tin  sulphide  may  separate;  it  can  be  dissolved  in 
ammonium  sulphide.  After  the  solution  is  completely  cool, 
any  sodium  sulphate  that  may  have  separated  is  dissolved 
by  addition  of  water  and  the  solution  electrolysed. 

The  determination  of  the  tin  is  much  more  simply  and 
easily  accomplished  by  converting  the  solution  of  tin  sulphide 
in  sodium  sulphide  into  the  acid  oxalate.  This  conversion 
may  be  accomplished  in  two  ways :  either  the  sulpho-salt  is 
decomposed  with  dilute  sulphuric  acid  to  remove  the  greater 
part  of  the  sulphur  as  hydrogen  sulphide,  and  the  separated 
tin  sulphide  oxidised  with  hydrogen  peroxide  *  until  the  stan- 
nic acid  which  is  produced  appears  pure  white,  or  the  heated 
alkaline  solution  is  treated  directly  with  hydrogen  peroxide 
(of  which  a  great  quantity  is  needed),  then  acidified  with  sul- 
phuric acid  to  precipitate  stannic  acid,  neutralised  with 
ammonia,  and  treated  with  more  hydrogen  peroxide.  In 
either  case  the  solution  is  heated  to  decompose  the  excess  of 
hydrogen  peroxide,  and  the  stannic  acid  allowed  to  settle 
and  then  filtered  off.  The  precipitate  is  washed  with  the 
oxalate  solution  from  the  filter  into  a  beaker,  the  filter  washed 
with  hot  oxalic  acid  solution,  and  the  stannic  acid  in  the 
beaker  dissolved  by  heating.  Sometimes  there  is  a  residue 
of  sulphur,  which  is  removed  by  filtration.  The  filtrate  is 
collected  in  the  weighed  platinum  dish  to  be  used  for  the  elec- 
trolysis, and  the  sulphur  is  washed  with  a  cold  saturated  solu- 
tion of  ammonium  oxalate  or  acid  ammonium  oxalate.  The 
solution  for  electrolysis  must  contain  at  least  4  g  of  the  oxalate. 

*  Classen  and  Bauer,  Ber.  d.  ch.  Ges.,  16,  1062  (1883). 


ARSENIC — PLATINUM.  217 


ARSENIC. 

LITERATURE ! 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  14  (1880). 

Classen  and  v.  Reiss,  Ber.  deutsch.  chem.  Ges.,  14,  1622  (1881). 

Moore,  Chem.  News,  53,  209  (1886). 

Yortmann,  Ber.  deutsch.  chem.  Ges.,  24,  2764  (1891). 

Ducru,  Compt.  rend.,  131,  886  (1900). 

Arsenic  cannot  be  quantitatively  separated  either  from 
aqueous  solutions  or  from  solutions  containing  hydrochloric 
acid,  ammonium  oxalate,  or  alkali  sulphides.  From  aqueous 
as  from  oxalic  acid  solutions  a  part  of  the  metal  is  reduced, 
while  from  hydrochloric  acid  solutions,  if  the  current  is  al- 
lowed to  act  for  a  sufficient  length  of  time,  all  of  the  arsenic 
passes  off  in  the  form  of  arseniuretted  hydrogen. 

The  behavior  of  arsenic  (prqsent  as  arsenic  acid)  in  a  con- 
centrated solution  of  sodium  sulphide  permits  the  separation 
of  arsenic  from  antimony,  as  will  be  shown  later. 


PLATINUM. 

LITERATURE: 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  13  (1880). 
Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2467  (1884). 
Smith,  Am.  Chem.  Journ.,  13,  206  (1891). 
Riidorff,  Zeit.  f.  angew.  Chem.,  p.  696  (1892). 

Platinum  is  very  readily  precipitated  from  its  solutions 
by  the  electric  current.  According  to  the  determinations 
made  by  Dr.  "W.  Gobbels  in  the  Aachen  laboratory,  if  a  solu- 
tion of  a  platinum  salt  containing  2-3  per  cent,  by  volume 
of  sulphuric  acid  is  used  and  is  electrolysed  with  a  current  of 
ND100  =  0.1-0.2  ampere,  all  the  platinum  separates  in  a  short 


218  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

time  in  the  form  of  platinum-black.  If,  however,  a  solu- 
tion heated  to  60-65°  is  electrolysed  with  a  current  of  ND100 
=  0.05  amp.  and  a  potential-difference  of  1.2  volts  the  plat- 
inum separates  quantitatively  and  in  a  very  compact  form. 
The  reduced  metal  is  so  dense  that  it  cannot  be  distinguished 
from  hammered  platinum.  The  operation  requires  a  period 
of  from  4  to  5  hours.  If  it  is  desired  to  remove  the  deposited 
platinum,  the  cathode  should  be  first  plated  with  a  coating 
of  copper  or  silver  (see  Zinc). 

According  to  the  method  employed  in  the  Munich  labo- 
ratory, if  the  quantity  of  platinum  present  is  about  0.4  g, 
2  per  cent,  by  volume  of  dilute  sulphuric  acid  (1 :  5)  is  added 
to  the  solution  of  the  platinum  salt,  the  liquid  is  heated  to 
about  65°,  and  electrolysed  with  a  current  of  ND10G  =  0.01  to 
0.03  ampere.  The  potential-difference  when  the  current- 
density  is  0.03  ampere  is  at  first  about  0.05  volt,  but  rises 
toward  the  end  of  the  reaction  to  1.7  volts.  Although  the 
odor  of  chlorine  escaping  at  the  anode  can  be  detected 
throughout  the  entire  operation,  no  evidence  has  been  ob- 
tained that  any  platinum  passes  into  solution  from  the  anode. 
The  precipitation  is  complete  in  about  5  hours. 

Indium  is  not  reduced  from  its  solutions  by  a  current  of 
ND100  =  0.05  amp.  and  1.2  volts  potential:  this  property  may 
be  used  for  the  quantitative  separation  of  platinum  from 
indium  (Classen). 

PALLADIUM. 

LITERATURE ! 

Wohler,  Lieb.  Ann.,  133,  357  (1865). 
Schucht,  Zeit.  f  anal.  Chem.,  22,  242  (1883). 
Smith  and  Keller,  Am.  Chem.  Journ.,  12,  252  (1890). 
Smith,  ibid.,  13,  206  (1891);  14,  435  (1892); 
Zeit.  f.  anorg.  Chem.,  3,  476  (1893). 
Joly  and  Leidi6,  Compt.  *end.,  116,  146  (1893). 


MOLYBDENUM.  219 

Palladium  is  determined  in  the  same  way  as  platinum. 
If  the  current  of  ND100  =  0.05  ampere,  with  a  potential- 
difference  of  1.2  volts,  is  used  for  the  reduction,  the  palla- 
dium is  obtained  in  an  excellent  metallic  condition. 


MOLYBDENUM. 

LITERATURE: 

Smith,  Am.  Chem.  Journ.,  i,  329  (1879). 

Hoskinson  and  Smith,  Am.  Chem.  Journ.,  7,  90  (1885). 

Kollock  and  Smith,  Journ.   Am.  Chem.  Soc.,  23,  669  (1901). 

According  to  Kollock  and  Smith  the  quantitative  deter- 
mination of  this  element  by  electrolysis  is  possible  under 
the  following  conditions: 

To  125  cc  of  a  solution  of  ammonium  molybdate  (con- 
taining the  equivalent  of  0.1302  g.of  molybdenum  tri oxide) 
two  drops  of  concentrated  sulphuric  acid  were  added,  and 
the  solution  was  electrolysed  at  75°  with  a  current-density 
of  ND100  =  0.04  ampere  and  a  potential-difference  of  2.2  volts. 
The  molybdenum  separated  as  black  hydrated  sesquioxide 
on  the  cathode.  The  precipitation  was  complete  in  about 
3  hours.  The  deposit  was  washed  without  interrupting  the 
current,  and  while  still  moist  was  dissolved  in  dilute  nitric 
acid,  the  solution  evaporated  to  dryness,  and  the  residue 
gently  heated  on  an  iron  plate  to  expel  the  last  traces  of  the 
nitric  acid.  White  molybdic  acid  remained.  If  blue  spots 
appeared  in  the  mass,  they  were  removed  by  a  second  treat- 
ment with  nitric  acid.  The  analytical  results  showed  a 
close  agreement  with  theory. 

The  authors  state  that  the  method  is  accurate,  is  easy  of 
execution,  and  requires  comparatively  little  time. 


220  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


POTASSIUM,  AMMONIUM.    (NITROGEN.) 

Potassium  and  ammonium  may  be  determined,  as  is  well 
known,  by  converting  them  into  potassium  or  ammonium 
platinchloride,  and  weighing  the  precipitate,  dried  at  110°, 
on  a  tared  filter.  This  method,  which  is  almost  universally 
employed  in  the  separation  of  potassium  from  sodium,  has 
many  disadvantages.  It  is  preferable,  after  precipitating 
and  washing  the  platinum  salt  as  usual,  to  dissolve  it  in  water, 
and  determine  the  platinum  as  directed  on  p.  218. 


DETERMINATION   OF   NITRIC  ACID   IN  NITRATES.        221 


SECTION  II. 

DETERMINATION  OF  NITRIC  ACID  IN  NITRATEa 

LITERATURE ! 

Luckow,  Zeit.  f.  analyt.  Chem.,  19,  11  (1880). 
Vortmann,  Ber.  deutsch.  chem.  Ges.,  23,  2798  (1890). 
Ulsch,  Zeit.  f.  Elektrochem.,  3,  546  (1896-97). 

As  is  well  known,  nitric  acid  is  often  converted  into  am- 
monia, and  the  latter  determined.  The  action  of  the  electric 
current  converts  nitric  acid  into  ammonia,  as  explained  in 
the  Introduction  (p.  38).  If  the  solution  of  an  alkali  nitrate, 
acidified  with  dilute  sulphuric  acid,  is  acted  on  by  the  cur- 
rent, no  ammonia  is  formed. 

Luckow  discovered  that  the  reduction  of  the  nitric  acid 
always  takes  place  when  a  salt  from  which  the  metal  is  pre- 
cipitated by  the  current  is  also  present  in  the  solution.  Cop- 
per salts  are  best  adapted  for  this  purpose.  G.  Vortmann 
has  determined  hi  the  Aachen  laboratory  the  conditions  for 
the  quantitative  determination  of  nitric  acid  in  nitrates. 
The  solution  of  the  nitrate  is  treated  with  a  sufficient  quantity 
of  copper  sulphate  (in  the  analysis  of  potassium  nitrate, 
half  as  much  crystallised  copper  sulphate  as  potassium  ni- 
trate), acidified  with  dilute  sulphuric  acid,  and  electrolysed 
cold.  When  the  reaction  is  complete  the  solution  is  poured 
off,  sodium  hydroxide  solution  is  added,  and  the  ammonia 
distilled  off  and  determined  volumetrically  in  the  usual  way. 
For  this  purpose  one-fifth  normal  solutions  of  ammonia  and 
sulphuric  acid  are  used.  To  standardise  the  sulphuric  acid, 
a  weighed  quantity  (0.5  g)  of  crystallised  copper  sulphate  is 
decomposed  by  electrolysis,  and  the  resulting  free  acid 


222  QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS. 

titrated  with  ammonia.  Vortmann  decomposed  0.4876  g 
CuS04.5H20,  and  used,  for  the  neutralisation  of  the  acid 
set  free,  19.6  cc  of  ammonia  of  a  strength  equal  to  the  one- 
fifth  normal  sulphuric  acid.  1  cc  of  the  latter  corresponds 
therefore  to  0.0028017  g  of  nitrogen  in  the  form  of  ammonia. 
The  method  described  by  Ulsch  is  based  on  the  reduction 
of  the  nitric  acid,  set  free  by  adding  sulphuric  acid  to  a  solu- 
tion of  nitrates,  when  the  electrolysis  is  conducted  with  a 
copper  cathode.  For  further  details  consult  the  original 
article. 


DETERMINATION    OF   THE    HALOGENS.  223 


SECTION  III. 

DETERMINATION  OF  THE  HALOGENS. 

Chlorine,  Bromine,  Iodine. 
LITERATURE  I 

Vortmann,  Monatshefte  f.  Chem.,  15,  280  (1894);  16,  674  (1895); 

Elektrochem.  Zeit,,  i,  137  (1894). 
Whitefield,  Am.  Chem.  Journ.,  8,  421  (1897). 
Specketer,  Zeit.  f.  Elektrochem.,  4,  539  (1897-98). 
Miiller,  Ber.  deutsch.  chem.  Ges.,  35,  950  (1902). 

The  method  originated  by  Vortmann  depends  upon  the 
principle  that  the  halogens  are  set  free  from  solutions  of  hal- 
ogen salts  by  the  electric  current,  and  while  in  the  ion  state 
combine  with  a  silver  anode  to  form  insoluble  silver  halide. 
The  increase  in  weight  of  the  anode  gives  directly  the  quan- 
tity of  halogen  which  has  separated.  The  completion  of  the 
analysis  is  determined  by  replacing  the  original  silver  anode 
by  a  second  weighed  silver  anode  and  noting  its  increase  in 
weight. 

For  an  experimental  test  of  the  method,  a  weighed  quan- 
tity of  iodide  is  dissolved  in  water,  6-10  cc  of  a  10%  solution 
of  sodium  hydroxide  added,  and  the  solution  diluted  to  100- 
150  cc.  The  silver  anode,  having  the  form  of  a  watch-glass 
6  cm  in  diameter,  is  fixed  about  5  mm  from  the  bottom  of 
the  copper  dish  which  serves  as  cathode. 

The  cold  solution  is  electrolysed  with  a  current-strength 
of  0.03-0.07  ampere  and  a  difference  of  potential  of  2  volts. 
After  4-5  hours  the  greater  part  of  the  iodine  will  be  converted 
into  silver  iodide,  and  the  remainder  may  be  separated  on  a 


224  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 

fresh  silver  anode,  after  the  addition  of  sodium  potassium  tar- 
trate  to  the  solution.  The  liquid  is  warmed  to  50-70°  and 
electrolysed  with  a  current  having  a  difference  of  potential  of 
1.2-1.3  volts  and  a  current-strength  of  0.01-0.02  ampere. 


SEPARATION    OF    METALS.  225 


SECTION  IV. 

SEPARATION    OF   METALS. 

IRON. 
Iron— Cobalt. 

LITERATURE  *. 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Engels,  Compt.  rend.,  15,  5,  20  (1896). 
Ducru,  Bull.  Soc.  Chim.,  17,  881  (1897). 
Neumann,  Chem.  Ztg.,  22,  731  (1898). 

The  two  metals  may  be  determined  by  electrolysing  the 
solution  of  the  double  oxalates,  as  directed  under  Iron  (p.  155), 
weighing  the  iron  and  cobalt  together,  and  determining  the 
former  volumetrically. 

After  weighing  the  iron  and  cobalt,  the  deposit  is  cuV 
solved  in  dilute  sulphuric  acid  (dilute  sulphuric  acid  is  poured 
over  the  metals,  and  concentrated  acid  gradually  added,  so 
that  the  solution  becomes  heated),  and  the  iron  is  titrated  in 
the  platinum  dish  with  potassium  permanganate.  To  over- 
come the  red  color  of  cobalt  sulphate,  a  sufficient  amount  of 
nickel  sulphate  is  added  before  the  titration.  The  end  of 
the  reaction  is  easily  recognised. 

The  residue  of  cobalt  and  iron  may  also  be  dissolved  in 
hydrochloric  acid,  the  iron  oxidised  with  hydrogen  peroxide, 
and  titrated  with  stannous  chloride,  after  removing  the  excess 
of  hydrogen  peroxide  by  boiling. 


226  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


EXPERIMENT. 


Used  1  g  each  of 
3K2C204.6H20,  and  8 
liquid,  120  cc. 

Current-       Electrode 
density,       Potential,       Temp. 
Amperes.          Volts. 

2.0-1.6     3.0-3.6     65-70° 


1.55-1.4   3.2-3.6     62-65c 


1.0-0.85    2.85-3.1  60-65° 


0.5-0.4      2.0-2.7     60-67° 


0.5-0.45   2.35-2.7  58-62° 


CoS04.K2S04.6H20  and    Fe2(C204)3. 
g  ammonium  oxalate.     Volume   of 


Found, 

Fe  +  Co.          Calculated.  Titrated, 

g 

1    40     0.2658     0.1141gFe*     0.1140gFe 
0.1517"  Co 
0.2658g 

1    20     0.2650     0.1138gFe       0.1140"" 
0.1517"  Co 
0 . 2b55  g 

230      0.2585    0.1137gFe      0.1140"" 
0.1451  "Co 

0.2586g 

4   —     0.2593     0.1136gFe      0.1133"" 
0.1452"  Co 
0.2588g 

4—     0.2617     0.1139gFe       0.1141"" 
0.1477"  Co 
0.2616g 


Iron — Nickel. 

LITERATURE : 

Vortmann,  Monatshefte  f.  Chem.,  14,  536  (1893); 

Elektrochem.  Zeit.,  p.  6  (1894). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Engels,  Compt.  rend.,  15,  5,  20  (1896). 
Ducru,  Bull.  Soc.  Chim.,  17,  881  (1897). 
Neumann,  Chem.  Ztg.,  22,  731  (1898). 

The  method  of  determination  is  exactly  like  the  preceding. 
Iron  and  nickel  separate  in  the  form  of  a  beautiful  white 
alloy  scarcely  distinguishable  from  platinum.  This  alloy  re- 

*  The  numbers  placed  under  the  heading  "Calculated"  are  the  quanti- 
ties of  iron  and  cobalt  in  the  two  salts  taken,  which  were  separately  deter- 
mined by  electrolysis. 


IRON. 


227 


sists  strongly  the  action  of  acids,  and  is  only  very  slowly 
attacked  by  dilute  sulphuric  or  hydrochloric  acid. 

Since  the  precipitation  of  the  last  trace  of  nickel  takes 
place  very  slowly,  the  use  of  a  current  of  at  least  ND100  =  1 
ampere  is  to  be  recommended.  Toward  the  end  of  the  opera- 
tion the  current-strength  should  be  increased. 

To  determine  the  iron,  the  precipitate  in  the  dish  must  be 
heated  with  concentrated  hydrochloric  acid;  and  if  the  iron 
is  to  be  titrated  with  permanganate,  the  solution  must  be 
reduced  by  nascent  hydrogen.  It  is  more  simple  to  oxidise 
with  hydrogen  peroxide,  and,  after  removing  the  excess,  tit- 
rate the  ferric  chloride  with  stannous  chloride. 


EXPERIMENT. 

Used  1  g  each  of  NiS04.(NH4)2S04.6H2O  and  Fe2(C204)3. 
3K2C204.6H2O,  and  8  g  ammonium  oxalate.  Volume  of 
liquid,  120  cc. 


Amperes. 
2.2-1.75 


Electrode 

Potential, 

Volts. 

3.45-4.0 


Temp. 
70-65° 


Time,        Found, 
hr.  m.       Fe  +  Ni. 


Calculated. 


2  — 


0.2760g      0.1135g  Fe* 
0.1622"  Ni 


2.0-1.75       3.35-3.9       69-67°       2  —       0.2654 


1.1-0.7 


2.6-3.1 


65-71°       4  30      0.2675 


0.5-0.4         2.6-3.0         68-71°       5  —       0.2664 


0.2757g 

0.1135gFe 
0.1527"^ 
0.2662g 

0.1135gFe 
0.1550"Ni 
0.2683g 
0.2664g 


Vortmann  adds  4-6  g  sodium  potassium  tartrate  and  au 
excess  of  sodium  hydroxide  to  the  solution,  warms,  and  pre- 

*  The  numbers  placed  under  the  heading  " Calculated"  are  the  quanti- 
ties of  iron  and  cobalt  in  the  two  salts  taken,  which  were  separately  deter- 
mined by  electrolysis. 


228  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

cipitates  the  iron  with  a  current  of  ND100  =  0.3-0.5  ampere  in 
three  to  four  hours,  the  nickel  remaining  in  solution. 

Iron — Zinc. 

LITERATURE  I 
Vortmann,  Monatshefte  f.  Chem.,  14,  536  (1893).  , 

If  the  double  oxalates  of  iron  and  zinc  are  submitted  to 
electrolysis,  the  two  metals  do  not  separate  as  an  alloy,  but 
zinc,  with  a  little  iron,  is  first  precipitated  on  the  negative  elec- 
trode.. The  electrolysis  proceeds  very  satisfactorily,  and  the 
united  weight  of  the  two  metals  may  readily  be  determined 
if  there  is  less  than  one-third  as  much  zinc  as  iron  in  the  solu- 
tion. If  the  proportion  of  zinc  is  greater,  the  zinc  dissolves 
with  the  evolution  of  gas  as  the  action  proceeds,  and  a  pre- 
cipitate of  oxide  of  iron  is  formed. 

Vortmann  proposes  the  following  method:  4-6  grams  of 
potassium  sodium  tartrate  and  an  excess  of  a  10-20%  solu- 
tion of  sodium  hydroxide  are  added  to  the  solution  of  the 
metals,  and  the  liquid  is  electrolysed  at  a  potential-difference 
of  2  volts,  with  a  current-strength  of  ND100  =  0.07-0.1  ampere, 
It  is  best  to  raise  the  temperature  at  the  close  of  the  operation 
to  50-60°.  After  several  hours  the  iron  will  be  precipitated, 
the  zinc  remaining  in  solution.  The  zinc  can  then  be  quan- 
titatively precipitated  by  electrolysing  with  a  potential-dif- 
erence  of  4  volts. 

Iron — Manganese. 

LITERATURE  *. 

Classen,  Ber  deutsch.  chem.  Ges.,  18,  1787  (1885). 
Engels,  Compt.  rend.,  pp.  5,  40  (1896). 
Kaeppel,  Zeit.  f.  anorg.  Chem.,  16,  268  (1898). 

As  stated  in  the  Introduction,  a  solution  of  ammonium 
oxalate  is  decomposed  by  electrolysis,  mainly  into  hydrogen 


IRON.  229 

and  hydrogen  ammonium  carbonate.  The  latter  is  partly 
decomposed  into  ammonia,  most  of  which  remains  in  solution, 
and  carbon  dioxide.  In  the  electrolysis  of  a  hot  solution  of 
ammonium  oxalate,  the  ammonium  carbonate  produced  by 
the  current  is  partly  neutralised  as  a  result  of  dissociation  of 
ammonium  oxalate;  carbon  dioxide  is  rapidly  liberated  at 
the  positive  electrode. 

If  a  solution  of  the  double  oxalates  of  iron  and  manga- 
nese is  subjected  to  electrolysis  without  the  previous  addition 
of  a  great  excess  of  ammonium  oxalate,  the  characteristic 
color  of  permanganic  acid  appears  immediately  at  the 
anode.  Manganese  dixode  gradually  separates  at  the  posi- 
tive electrode,  and  iron  at  the  negative.  If  the  electrolysis 
is  conducted  under  these  conditions,  it  is  impossible  to  obtain 
a  quantitative  separation  of  the  two  metals,  since  the  manga- 
nese dioxide  carries  down  with  it  considerable  quantities  of 
ferric  hydroxide.  The  complete,  separation  of  the  metals  is 
possible  only  when  the  separation  of  the  manganese  dioxide 
is  delayed  till  most  of  the  iron  is  precipitated.  If  a  solution  of 
the  double  oxalates  of  iron  and  manganese,  which  contains  a 
great  excess  of  ammonium  oxalate,  is  electrolysed  in  the  cold, 
the  greater  part  of  the  manganese  dioxide  is  precipitated  only 
after  most  of  the  ammonium  oxalate  is  decomposed.  In  this 
case,  however,  the  separation  of  the  manganese  dioxide  is  in- 
complete, because  by  the  action  of  the  current  a  considerable 
quantity  of  ammonium  carbonate  or  ammonia  is  produced 
which  acts  on  the  manganese  double  salt,  causing  a  portion  of 
the  precipitate  (a  mixture  of  dioxide  and  a  lower  oxide)  to  pass 
into  solution. 

The  rapid  decomposition  of  ammonium  oxalate  when 
heated  gives  a  simple  means  of  delaying,  or  entirely  preventing, 
the  formation  of  a  manganese  precipitate  during  electrolysis. 

The  double  oxalate  is  prepared  by  the  method  given  under 


230  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Iron,  with  the  difference  only  that  8  to  10  g  ammonium  oxa- 
late  are  dissolved  in  the  liquid,  which  is  warmed  to  80-90°, 
and  electrolysed  with  a  current  of  ND100  =  0.5  amp. 

When  the  reduction  is  complete,  the  solution  is  poured 
off,  the  dish  washed  repeatedly  with  water,  and  this,  together 
with  traces  of  the  dioxide  precipitate,  removed  by  alcohol ;  it 
is  sometimes  necessary  to  rub  the  dish  gently  with  the  finger. 

The  preceding  method  gives  satisfactory  results  when  the 
percentage  of  manganese  is  not  too  high.  For  the  analysis 
of  manganiferous  iron  (ferro-manganese,  for  example)  this 
method  has  no  practical  value,  since  the  percentage  of  man- 
ganese is  here  required,  while  by  this  method  the  iron  is 
determined  directly  and  the  manganese  must  be  determined 
in  the  liquid  from  which  the  iron  has  been  separated. 

To  obtain  a  complete  separation,  the  solution,  containing 
suspended  manganese  dioxide,  is  heated  with  a  solution  of 
pure  potassium  or  sodium  hydroxide  in  a  porcelain  dish, 
till  the  ammonium  carbonate  produced  by  electrolysis  is  de- 
composed and  the  solution  no  longer  has  the  odor  of  ammo- 
nia; and  then  sodium  carbonate  and  a  small  quantity  of  so- 
dium hypochlorite,  or,  better,  hydrogen  peroxide,  are  added. 
The  manganese  dioxide  quickly  falls  to  the  bottom,  and  can 
be  filtered  off.  The  precipitate  is  best  washed  with  hot  water 
to  which  a  little  ammonium-nitrate  has  been  added,  and  is 
either  converted  into  mangano-manganic  oxide  (Mn2O4)  by 
ignition,  or,  better,  into  manganese  sulphate  (MnSOJ. 

The  conversion  into  manganese  sulphate  is  accomplished 
by  moistening  the  precipitate  in  the  crucible  with  a  little  pure 
concentrated  sulphuric  acid,  and  igniting  very  gently,  so  that 
the  bottom  of  the  crucible  is  heated  to  a  low  red  heat. 

If  it  is  desired  to  determine  the  manganese  as  manganese 
sulphide,  the  solution  is  boiled  till  the  ammonium  carbonate 
is  decomposed,  the  remaining  ammonia  is  neutralised  with 


IRON.  231 

nitric  acid,  and  ammonium  sulphide  added  till  the  precipi- 
itation  is  complete.  The  manganese  sulphide  is  either  deter- 
mined as  such  by  ignition  in  a  stream  of  hydrogen,  or,  more 
simply,  converted  into  manganese  sulphate  by  heating  with  a 
few  drops  of  sulphuric  acid. 

Kaeppel  conducts  the  separation  of  iron  from  manganese 
in  a  solution  of  the  double  salts  of  pyrophosphoric  acid.  A 
solution  of  ferric  ammonium  sulphate  (0.1  to  0.15  g  Fe)  and 
manganous  ammonium  sulphate  (0.035  to  0.11  g  Mn)  was 
added  with  constant  stirring  to  a  boiling  solution  containing 
12  g  sodium  pyrophosphate,  and  when  the  solution  had  be- 
come clear  five  drops  of  phosphoric  acid  were  added.  If  the 
phosphoric  acid  produced  turbidity,  this  was  removed  by 
adding  a  few  drops  of  sodium  pyrophosphate  solution.  The 
total  volume  of  the  solution  was  230-250  cc,  and  the  elec- 
trolysis was  conducted  in  a  platinum  dish  which  served  as 
anode  with  a  current  of  1.8  to  2.5  ampere  (sic.),  at  a  potential- 
difference  of  4  volts.  The  temperature  of  the  electrolyte  was 
30-40°,  and  the  time  required  was  8  to  10  hours;  the  iron 
being  precipitated.  For  the  iron  fairly  accurate  results  were 
obtained,  but  the  manganese  remaining  in  the  solution  could 
not  be  determined  by  electrolytic  methods.  The  deposited 
iron  was  washed  without  interrupting  the  current. 

Iron — Aluminium. 

LITERATURE: 

Classen,  Ber.  deutsch.  chem.  Ges.,  18,  1795  (1885);  27,  2060  (1894). 
Engels,  Compt.  rend.,  15,  5,  20  (1896). 

When  a  solution  containing  the  above  metals  and  a  great 
excess  of  ammonium  oxalate  is  electrolysed  in  the  cold,  iron 
is  deposited  on  the  negative  electrode,  while  the  aluminium 
remains  in  solution  as  long  as  ammonium  oxalate  is  present 
in  the  solution  in  greater  proportion  than  the  ammonium 


232  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

carbonate  formed  from  it.  If  a  precipitate  of  aluminium 
hydroxide  finally  appears,  it  is  only  when  the  solution  is  al- 
most free  from  iron.  A  small  portion  withdrawn  by  a  capil- 
lary tube  is  tested,  from  time  to  time,  with  ammonium  sul- 
phide or  another  reagent  already  mentioned,  and  the  current 
is  stopped  as  soon  as  no  reaction  is  obtained. 

The  process  is  as  follows:  The  aqueous  or  weakly  acid 
solution  (in  the  latter  case  neutralised  with  ammonia)  of  the 
sulphates  (the  chlorides  are  not  as  well  adapted  to  the  process) 
is  treated  with  ammonium  oxalate  in  excess,  and  enough  solid 
ammonium  oxalate  added  (with  gentle  warming  if  necessary) 
to  give  the  proportion  of  2-3  g  ammonium  oxalate  to  0.1  g 
of  the  metals.  The  entire  volume  of  the  solution  should  be 
150-175  cc.  If  the  temperature  of  the  solution  is  not  over 
40°,  it  may  be  submitted  to  electrolysis  at  once,  since  it  grad- 
ually cools  under  the  action  of  a  current  of  the  given  strength. 

It  is  not  best  to  continue  the  action  of  the  current  longer 
than  is  necessary  to  reduce  the  iron;  for,  otherwise,  a  large 
part  of  the  aluminium  is  precipitated  as  hydroxide,  and 
clings  so  closely  to  the  negative  electrode  that  it  cannot  be 
removed. 

In  such  a  case  it  is  necessary  to  bring  the  aluminium  hy- 
droxide into  solution  by  acidifying  with  oxalic  acid,  and,  in 
case  too  much  acid  has  been  added,  to  pass  the  current  till  the 
last  traces  of  the  redissolved  iron  have  been  again  precipitated. 

The  oxalic  acid  is  poured  gradually  down  the  glass  which 
covers  the  platinum  dish,  without  interrupting  the  current, 
till  there  is  no  more  ebullition,  and  the  aluminium  precipitate 
is  dissolved. 

If  the  quantity  of  the  aluminium  is  not  greater  than  that  of 
the  iron,  the  method  gives  good  results  without  further  treat- 
ment. In  other  cases  the  precipitate  of  aluminium  hydrox- 
ide is  dissolved,  without  interrupting  the  current,  by  careful 


IRON. 


233 


addition  of  oxalic  acid,  and  the  electrolysis  repeated  until 
the  iron  is  completely  precipitated.  To  determine  the  alu- 
minium in  the  solution  poured  off  from  the  iron,  it  is  heated 
in  a  porcelain  dish  till  the  ammonia  is  driven  off,  filtered, 
and  the  aluminium  hydroxide  converted,  by  ignition,  into 


A1203. 


EXPERIMENT. 


Used  1  g  each  of  Fe2(C204)3.3K2C204.6H20  and  A12(S04)3. 
K2S04.24H2O,  and  8  g  ammonium  oxalate.  Volume  of 
liquid,  120  cc. 


Current- 
density, 
Amperes. 

Electrode 
Potential, 
Volts. 

Temp. 

Time, 
hr.   m. 

Taken, 

g- 

Found, 
g. 

1.95-1.6 

4.3  -4.4 

31-42° 

2  35 

0.1135Fe 

0.1143Fe 

1.65-1.35 

3.8  -4.1 

30-48° 

3  — 

0.1150" 

0.1159  " 

1.00-0.84 

3.55-3.8 

31-36° 

4  30 

0.1135  " 

0.1138  " 

0.50-0.42 

2.75-3.1 

30-32° 

5  40 

0.1135" 

0.1139  " 

In  order  to  avoid  the  separation  of  aluminium  hydroxide 
(small  quantities  of  which  often  adhere  to  the  iron)  strong 
currents,  which  raise  the  temperature  of  the  solution,  should 
not  be  used. 

The  effect  of  strong  currents  and  high  temperatures  is 
illustrated  in  the  above  experiment. 

Iron — Chromium. 

LITERATURE  I 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Engels,  Compt.  rend.,  15,  5,  20  (1896). 

If  a  solution  which  contains  an  excess  of  ammonium  oxa- 
late, and  chromium  as  sesquioxide,  that  is,  as  chromium 
ammonium  oxalate,  be  submitted  to  electrolysis,  all  of  the 
chromium  is  converted  into  a  chromate.  If  iron  is  also  pres- 
ent, it  is  precipitated  in  the  metallic  state  on  the  negative 
electrode;  the  metal  has  a  peculiarly  characteristic  luster. 


234  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

When  the  precipitation  is  complete,  the  liquid  is  poured 
off  from  the  precipitated  metal  and  is  boiled  to  decompose 
ammonium  carbonate,  and  the  chromic  acid  reduced  by  boil- 
ing with  hydrochloric  acid  and  alcohol.  The  chromium  is 
then  precipitated  as  hydroxide  with  ammonia. 

The  hydroxide  is  converted  into  Cr203in  the  usual  manner, 
and  weighed. 

EXPERIMENT.      . 

A.  Used  1  gram  of  Fe2(C204)3.3K2C204.6H2,  1  gram  of 
3K2C204.Cr2(C204)3.6H2O,  and  8  grams  ammonium  oxalate. 
Solution  diluted  to  120  cc. 


Current- 
density, 
Amperes. 

Electrode 
Potential, 
Volts. 

Temp. 

Time, 
hr.  m. 

Taken, 
Fe. 

Found, 
Fe. 

2.00-1.60 

3.4-3.6 

62-68° 

4  - 

0.1120g 

0.1123g 

1.60-0.95 

3.2-3.8 

66-68° 

5  — 

0.1135" 

0.1135" 

1.95-1.50 

3.3-3.7 

62-65° 

3  — 

0.1135" 

0.1130" 

B.  Used  2  g  chrome  alum,  1.5890  g  ferrous  ammonium 
sulphate,  and  8  g  ammonium  oxalate. 

Current-  Electrode  T;™,  ToL-^n  v        ^ 

density,  Potential,  Temp.          Time,  Taken,  Found, 

Amperes.  Volts. 

1.5  3  65°          4  —  14.28%  14.19% 

C.  Used  2  g  chrome  alum,  1  g  Fe2(C204)3.3K2C204.6H20, 
and  8  g  ammonium  oxalate, 


1.50-1.60      3.0-3.2          65°          4    —       11.40%          11.35% 
Iron  —  Uranium. 

The  separation  of  iron  from  uranium  depends  upon  the  same 
principle  as  the  separation  of  iron  from  aluminium.  It  is  nec- 
essary to  have  a  great  excess  (8  g)  of  ammonium  oxalate  pres- 
ent in  the  solution,  in  order  to  retain  the  uranium  in  the  form 
of  the  double  salt  until  all  of  the  other  metal  is  precipitated. 


IRON.  235 

The  process  is  conducted  in  the  same  manner  as  the  separa- 
tion of  aluminium  from  iron.  .When  a  strong  current  is 
employed,  especially  when  there  is  an  insufficient  quantity  of 
ammonium  oxalate  present,  it  may  happen  that,  as  a  result  of 
the  decomposition  of  the  hydrogen  ammonium  carbonate  by 
the  heat  produced,  the  uranium  is  precipitated  as  hydroxide. 

The  uranium  solution,  after  the  other  metal  has  been 
separated,  is  freed  from  oxalic  acid  by  further  electrolysis, 
and  finally  the  ammonium  carbonate  is  decomposed  by 
heating. 

To  bring  the  finely  divided  precipitate  of  uranium  hydrox- 
ide into  suitable  condition  for  filtration,  nitric  acid  is  added, 
the  solution  is  heated  till  the  precipitate  is  wholly  dissolved, 
and  ammonia  is  added  to  reprecipitate  the  hydroxide.  The 
precipitate  is  converted  into  uranium  oxide  by  ignition  in  a 
stream  of  hydrogen. 

Iron — Aluminium — Chromium. 

LITERATURE  I 
Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771  (1881). 

The  separation  is  performed  as  above.  To  separate  the 
aluminium  from  chromium,  the  solution  poured  off  from  the 
precipitated  metals  is  boiled  till  it  has  only  a  weak  -odor  of 
ammonia,  the  aluminium  hydroxide  filtered  off,  and  the  chro- 
mium precipitated  as  above. 

Iron — Chromium — Uranium. 

LITERATURE 

Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771  (1881); 

ibid.  17,  2483  (1884). 

The  separation  is  accomplished  by  the  precipitation  of 
iron  as  metal,  from  the  double  oxalate  solution,  and  the  oxi- 
dation of  chromium  to  chromic  acid  by  the  current.  Uran- 
ium is  separated  as  hydroxide,  while  chromium  remains  in 


236  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

•solution  as  ammonium  chromate.  To  accomplish  the  quan- 
titative separation  of  chromium  from  uranium  the  electrolysis 
must  be  continued  till  the  oxalic  acid  is  completely  oxidised. 
The  solution  is  boiled  to  decompose  the  resulting  am- 
monium carbonate,  and  allowed  to  stand  six  hours.  The  chro- 
mium is  determined,  as  above,  in  the  filtrate  from  the 
uranium. 

Iron —  Beryllium. 

LITERATURE  I 
Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771  (1881). 

The  separation  of  these  two  metals  offers  no  difficulties 
whatever  if  the  soluble  double  salts  with  ammonium  oxalate 
are  prepared,  and  if  care  is  taken  to  have  an  excess  of  ammo- 
nium oxalate  present.  The  iron  is  precipitated  according  to 
the  directions  given  under  the  separation  of  aluminium  from 
iron. 

Strong  currents  are  not  advisable  lest  the  solution  become 
heated,  and  thus  the  ammonium  carbonate,  which  holds  the 
beryllium  in  solution,  be  (decomposed.  The  beryllium  hy- 
droxide may,  in  any  case,  begin  to  precipitate  before  the  iron 
is  fully  deposited.  The  determination  of  beryllium  in  the 
solution  poured  off  from  the  iron  is  very  simple ;  the  solution 
is  boiled  to  decompose  the  hydrogen  ammonium  carbonate, 
and  the  heating  continued  till  the  solution  has  only  a  weak 
odor  of  ammonia.  The  beryllium  hydroxide  is  filtered, 
washed  with  hot  water,  and  converted  into  BeO  by  ignition 
in  a  platinum  crucible. 

Iron — Beryllium — Aluminium. 

LITERATURE : 
Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771  (1881). 

The  process  is  precisely  like  the  foregoing.  When  the 
iron  is  reduced  the  solution  is  poured  into  a  second  platinum 


IRON.  237 

dish,  and  the  action  of  the  current  is  continued  till  all  the 
oxalic  acid  is  decomposed,  and  the  aluminium  is  precipitated 
as  hydroxide.  The  beryllium  is  precipitated  from  the  filtrate 
as  hydroxide  by  boiling. 

It  is  advisable  to  redissolve  the  aluminium  hydroxide, 
to  convert  it  again  into  the  double  oxalate,  and  to  repeat  the 
electrolysis. 

Iron — Copper. 

LITERATURE  I 

Schweder,  Berg-  u.  Hiittenm.  Ztg.,  36,  5,  11,  31  (1877). 
Vortmann,  Monatshefte  f.  Chem.,  14,  536  (1893). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 

The  separation  may  be  accomplished  according  to  the 
method  given  by  Luckow  (p.  179),  if  the  operation  is  con- 
ducted at  ordinary  temperatures.  To  determine  the  iron  in 
the  solution  from  which  the  copper  has  been  removed  it  is 
evaporated  to  dryness  with  the  addition  of  sufficient  sulphuric 
acid  to  convert  the  iron  into  sulphate,  and  the  double  oxalate 
is  prepared  by  the  method  given  on,  page  154. 

EXPERIMENT. 

Used  about  1  g  each  of  copper  sulphate  and  ferrous  am- 
monium sulphate  and  5  cc  nitric  acid  (sp.  gr.  1.35).  Volume 
of  liquid,  120  cc. 

Current -density.       Electrode  T  Time,  Taken  Found 

Amperes.      Potential,  Volts.     J  hr.  m.  Cu.  Cu. 

1.0-0.9         3.0-3.3         19-32°       4-         0.2528g        0.2518g 
1.1-1.0         2.6-3.2         18-32°       330       0.2450"       0.2430" 

The  free  sulphuric  acid  in  the  decanted  liquid  was  neu- 
tralised with  ammonium  hydroxide,  and  8  g  ammonium 
oxalate  were  added. 

Current-density,  Electrode         Tom  Time,  Taken  Found 

Amperes.  Potential,  Volts.    A  hr.  m.  Fe.  Fe. 

1.30-0.8  2.7-4.5         31-42°       3  -        0.1406g      0.1416g 

1.45-1.1  3.0-3.5         60°  330       0.1435"    0.1438' 


238  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

A  similar  separation  may  also  be  carried  out  in  the  pres- 
ence of  sulphuric  acid  instead  of  nitric  acid.  Three  cubic 
centimeters  of  the  concentrated  acid  are  used,  the  other  con- 
ditions being  the  same. 

Current  -density,        Electrode  ™_  Time,  Taken  Found 

Amperes.         Potential,  Volts.   lemp'  hr.  m.  Cu.  Cu. 

1.05-1.20         3.0-2.85       22-30°       2  10       0.2539g      0.2534g 
1.00-0.95         2.5-2.45       56-59°        2-         0.2510"      0.2504" 

The  determination  of  the  iron  was  conducted  as  before. 

Current-density,        Electrode  T  Time,  Taken  Found 

Amperes.        Potential,  Volts.    x  hr.  m.  Fe.  Fe. 

1.55-1.32         3.4-3.8         33-40°        4-         0.1421g      0.1419g 
1.60-1.40         3.0-3.5         61-64°        3-         0.1675"     0.1625" 

The  separation  of  iron  and  copper  may  be  effected  if  the 
copper  is  precipitated  from  a  hot  solution  of  the  double 
oxalate  containing  free  oxalic,  tartaric,  or  acetic  acid.  A 
saturated  solution  of  oxalic  acid  is  used,  and  one  of  tartaric 
acid  which  contains  6  g  acid  in  every  100  cc. 

EXPERIMENT. 

Used  about  1  g  each  of  copper  sulphate  and  ferric  salt, 
6  g  ammonium  oxalate.  The  copper  must  be  washed  without 
interrupting  the  current. 

Current-density,  Electrode          TAT™         Ti™  Taken  Found 

Amperes.  Potential.  Volts    TemP-  Cu.  Cu. 

1.1-1.0  2.95-3.5         51-62°      3  hr.       0.2528g      0.2525g 

0.7-0.7  3.20-2.85       62°  3  "        0.2530"     0.2532" 

The  iron  was  determined  in  the  solution  which  was  poured 
off  from  the  copper,  the  free  acid  being  first  neutralised  with 
ammonium  hydroxide. 

Current-density,         Electrode  T  T-  Taken  Found 

Amperes.        Potential,  Volts.  J  Fe.  Fe. 

1.4-1.3  3.0-3.2  68-70°  2*  hr.  0.1435g  0.1431g 

10-0.9  3.1-3.3  30-40°  3     "  0.1429"  0.1425" 


IRON.  239 

Vortmann  dissolves  the  oxides  of  both  metals  in  an  am- 
moniacal  solution,  to  which  are  added  several  grams  of  ammo- 
nium sulphate,  and  electrolyses  with  a  current  of  ND100  = 
0.1-0.6  ampere.  Only  copper  is  precipitated,  the  ferric 
hydroxide  remaining  unaltered  in  solution. 

According  to  Fernberger  and  Smith,  copper  can  be  sepa- 
rated from  iron  under  the  conditions  given  in  the  following 
experiment : 

60  cc  of  a  solution  of  disodium-hydrogen  phosphate 
(sp.  gr.  1.0358)  were  added  to  a  mixture  of  25  cc  cf  a  copper 
sulphate  solution  (  =  0.1239  g  Cu)  and  50  cc  of  a  solution  of 
ferric  ammonium  alum  (  =  0.2002  g  Fe).  The  precipitate 
which  formed  was  dissolved  in  10  cc  phosphoric  acid  (sp.  gr. 
1.347),  and  the  solution  diluted  to  a  total  volume  of  225  cc 
was  electrolysed  at  53°  with  a  current  of  ND100  =  0.04  ampere 
and  a  potential-difference  of  2.4  volts.  The  copper  was 
completely  precipitated  in  about  7  hours. 

Iron — Lead. 

This  separation  is  effected  by  precipitating  the  lead  as 
peroxide  from  a  solution  containing  free  nitric  acid  (p.  195). 
To  determine  the  iron  remaining  in  the  solution  add  sulphuric 
acid,  evaporate  to  dryness  to  expel  the  nitric  acid,  and  de- 
termine the  iron  by  electrolysis  in  an  oxalic  acid  solution  as 
described  under  Iron-Copper. 

Iron — Bismuth. 

LITERATURE  I 
Kammerer,  Journ.  Am.  Chem.  Soc.,  25,  83  (1903). 

According  to  Kammerer  the  iron  should  be  present  as 
ferrous  sulphate,  and  the  solution  having  a  volume  of  150  cc 
should  contain  1  cc  nitric  acid  (sp.  gr.  1.43),  1  g  potassium 
sulphate,  and  2  cc  sulphuric  acid  (sp.  gr.  1.84).  The  elec- 


240  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

trolysis  is  conducted  at  45°  with  a  current  of  ND100  =  0.025 
ampere  and  a  potential-difference  between  the  electrodes  of 
2  volts.  About  8  hours  are  required  for  the  complete  sepa- 
ration of  the  bismuth. 

Iron — Cadmium. 

LITERATURE ! 
Stortenbeker,  Zeit.  f.  Elektrochem.,  4,  409  (1897-98). 

The  following  method  for  separating  these  two  elements 
is  given  by  Stortenbeker:  Cadmium  and  ferrous  sulphates 
are  dissolved  in  100  cc  of  water  slightly  acidified  with  a  few 
drops  of  dilute  sulphuric  acid,  and  to  this  solution  2  to  3 
grams  of  potassium  cyanide  are  added.  The  mixture  is  then 
warmed  until  perfectly  clear,  and  if  it  does  not  become  yel- 
low immediately  a  few  drops  of  a  solution  of  potassium  hy- 
droxide are  added.  The  solution  is  diluted  to  200  cc  and 
electrolysed  with  a  current  of  ND100  =  0.05  to  0.10  ampere, 
at  the  room  temperature.  Cadmium  will  be  deposited. 

Iron — Silver. 

.  i          LITERATURE: 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

The  following  experiment  is  described  by  Kollock:  To  a 
solution  containing  0.1  g  silver  as  nitrate  and  0.1  g  of  iron  as 
ferrous  ammonium  sulphate,  2.5-4  g  potassium  cyanide  were 
added  and  the  solution  was  diluted  to  125  cc.  The  elec- 
trolysis was  carried  out  at  65-75°  with  a  current  of  ND100  = 
0.02-0.04  ampere  and  a  potential-difference  of  1.8-2.2  volts. 
The  silver  was  completely  precipitated  in  from  3  to  4  hours. 


COBALT.  241 

COBALT. 
Cobalt— Nickel. 

LITERATURE:  * 

Vortmann,  Monatsh.  f.  Chemie,  14,  536  (1893). 

Cohen,  Zeit.  f.  Elektrochem.,  4,  501  (1897-98). 

Balachowsky,  Compt.  rend.,  132,  1492  (1901). 

Rosenheim  and  Huldschinsky,  Pharra.  Centralbl.,  42,  393  (1901). 

The  following  method  for  separating  these  two  elements 
is  described  by  Balachowsky :  To  an  acetic  acid  solution  con- 
taining 0.3  g  of  the  metals,  3  g  ammonium  sulpho-cyanide, 
1  g  urea,  and  enough  ammonia  to  neutralise  the  free  acetic 
acid,  are  added.  The  solution  is  diluted  to  a  convenient 
volume  and  electrolysed  at  70-80°  in  a  platinum  dish-elec- 
trode with  a  maximum  potential-difference  between  the 
electrodes  of  1.0  volt  and  a  current  of  ND100  =  0.8  ampere. 
The  precipitation  of  the  nickel  is  complete  in  H  hour,  but 
the  deposited  metal  contains  sulphur  and  should  therefore 
be  dissolved  in  nitric  acid,  the  sulphur  filtered  off,  and  the 
nickel  again  precipitated  by  one  of  the  standard  methods 
(p.  160).  The  cobalt  remaining  in  the  first  solution  is  de- 
termined by  destroying  the  ammonium  sulphocyanide  by 
boiling  with  nitric  acid,  filtering  off  any  sulphur  which  ap- 
pears, and  precipitating  the  cobalt  by  one  of  the  regular 
methods. 

The  method  proposed  by  Vortmann  depends  upon  the 
electrolysis  of  a  solution  containing  sodium  potassium  tar- 
trate  made  strongly  alkaline  with  sodium  hydroxide.  Vort- 
mann states  that  with  a  current-density  of  ND100  =  0.3  to  0.06 
ampere  the  nickel  remains  in  the  solution  while  the  cobalt  is 
precipitated.  The  essential  data  for  repeating  these  ex- 
periments are  lacking. 


242  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

Cobalt — Zinc. 

LITERATURE ! 

Vortmann,  Monatsch.  f.  Chemie,  14,  536  (1893); 

Elektrochem.  Zeit.,  i,  6  (1894). 

Smith  and  Wallace,  Journ.  of  Anal.  Chem.,  7,  183  (1893). 
Waller,  Zeit.  f.  Elektrochem.,  4,  241  (1897-98). 

According  to  Vortmann  an  excess  of  a  10-20%  solution 
of  sodium  hydroxide  is  added  to  the  solution  containing  the 
metals.  Several  grams  of  sodium  potassium  tartrate  are  then 
added  and  the  electrolysis  is  conducted  with  a  current  of 
ND100  =  0.07-0.1  ampere  and  an  electrode  tension  of  2  volts. 
The  cobalt  is  precipitated,  but  the  addition  of  potassium 
iodide  is  necessary  in  order  to  prevent  the  separation  of 
cobaltic  oxide  at  the  anode. 

According  to  Waller,  who  conducted  his  researches  in 
the  laboratory  of  the  author  at  Aachen,  satisfactory  results 
can  be  obtained  under  the  following  conditions : 

6  grams  of  sodium  potassium  tartrate  and  from  1.0  to 
1.5  g  of  potassium  iodide  were  dissolved  in  the  solution  con- 
taining the  metals  as  sulphates,  which  was  then  made  alka- 
line with  10  cc  of  a  solution  of  sodium  hydroxide  containing 
2  to  3  grams  of  NaOH.  The  solution  was  diluted  to  150  cc, 
and  was  electrolysed  at  60-65°  with  a  current  of  ND100  = 
0.05  to  0.1  ampere  and  a  potential-difference  of  2  volts. 
Since  a  little  oxide  of  cobalt  is  always  deposited  on  the  anode 
it  is  important  that  the  latter  should  be  weighed  both  before 
and  after  the  electrolysis,  in  order  to  determine  the  weight  of 
cobalt  thus  separated.  This  seldom  amounts  to  more  than 
1%  of  the  entire  quantity  present. 

The  zinc  is  afterwards  separated  from  the  solution  by 
electrolysing  with  a  potential-difference  of  4  volts. 


COBALT.  243 

Cobalt — Aluminium. 

The  method  is  carried  out  similarly  to  that  of  iron  from 
aluminium. 

Cobalt — Uranium;  .Cobalt — Chromium;    Cobalt — Uranium — Chromium. 

The  methods  employed  are  similar  to  those  of  the  corre- 
sponding separations  from  iron  (p.  234). 

Cobalt — Copper. 

LITERATURE: 

Warwick,  Zeit.  f.  anorg.  Chem.,  i,  299  (1892). 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  388  (1894). 

Fernberger  and  Smith,  Jour.  Am.  Chem.  Soc.,  21,  1001  (1899). 

The  separation  of  these  two  elements  can  be  satisfac- 
torily conducted  by  the  electrolysis  of  a  solution  containing 
ammonium  oxalate,  at  a  temperature  of  50-60°,  if  the  differ- 
ence of  potential  between  the  electrodes  is  so  regulated  as 
to  be  kept  between  1.1  and  1.3  volt.  Under  these  condi- 
tions only  copper  will  be  precipitated. 

EXPERIMENT. 

Used  1  g  copper  sulphate  (25.33%  Cu),  1  g  cobalt  ammo- 
nium sulphate,  and  6  g  ammonium  oxalate. 

Electrode  Potential .  T«««  Time,  Found 

Volts.  hr.     m.  gCu.  *Cu. 

1.24-1.30  50-60°  3  50  0.2602  25.36 
1.20-1.35  50-60°  3  30  0.2531  25.29 
1.20-1.29  50-60°  4  0.2522  25.28 

These  elements  may  also  be  separated  in  a  solution  con- 
taining free  nitric  acid  (p.  179). 

For  effecting  the  separation  in  a  solution  containing 
phosphates  we  are  indebted  to  Fernberger  and  Smith 


244  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

for  the  following  particulars:  To  a  solution  containing 
0.1329g  Cu  and  0.1  g  Co  in  the  form  of  sulphates,  60  cc  diso- 
dium  hydrogen  phosphate  (sp.  gr.  1.0358)  and  10  cc  phosphoric 
acid  (sp.  gr.  1.347)  were  added,  and  the  mixture  was  diluted 
to  a  volume  of  225  cc.  The  electrolysis  was  conducted  at 
62°  with  a  current  of  ND100  =  0.035  ampere  arid  a  potential- 
difference  of  1.5  volt.  The  copper  was  completely  de- 
posited in  6  hours. 

Cobalt — Bismuth. 

LITERATURE  I 

Smith  and  Wallace,  Journ.  of  Anal.  Chem.,  7,  183  (1893). 
Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  4,  268  (1893). 
Kammerer,  Journ.  Am.  Chem.  Ges.,  25,  83  (1903). 

According  to  Smith  and  Wallace,  and  also  Smith  and 
Moyer,  a  separation  of  these  metals  may  be  satisfactorily 
conducted  in  a  solution  containing  nitric  acid.  Since,  how- 
ever, the  required  conditions  of  experiment  are  not  given  in 
the  respective  publications  the  methods  will  be,  here  omitted. 

Kammerer  states  that  the  following  conditions  are  sat- 
isfactory for  effecting  this  separation : 

Cobalt  sulphate  in  quantity  equivalent  to  0.15  g  cobalt^ 
0.5  g  potassium  sulphate  and  2  cc  sulphuric  acid  (sp.  gr.  1.84) 
were  added  to  a  solution  containing  0.15  g  of  bismuth  and  2 
cc  nitric  acid  (sp.  gr.  1.43).  The  solution  was  diluted  to  150 
cc  and  electrolysed  at  a  temperature  of  45°  with  a  current  of 
ND100  =  0.025  ampere  and  a  potential-difference  of  2  volts. 
The  bismuth  was  completely  deposited  in  9  hours,  and  was 
free  from  cobalt.  This  method  serves  equally  well  for  the 
separation  of  bismuth  and  nickel. 

Cobalt— Lead. 

The  solution,  after  the  addition  of  nitric  acid,  is  electro- 
lysed as  described  under  Lead,  p.  195. 


NICKEL.  245 

Cobalt— Silver. 
LITERATURE. 
Kollock,  Journ.  Am.  Chem.  Soc,,  21,  911  (1899). 

The  following  experiment  is  described  by  Kollock:  To 
a  solution  containing  0.1  g  silver  and  0.1  g  cadmium,  both 
present  as  nitrates,  2.75  g  of  potassium  cyanide  was  added, 
and  the  solution  was  diluted  to  125  cc.  The  electrolysis  was 
carried  out  with  a  current  of  ND100  =  0.02  ampere  and  a  po- 
tential-difference of  2.2  to  2.7  volts.  After  5  hours  had 
elapsed  the  silver  was  completely  precipitated.  The  tem- 
perature of  the  solution  throughout  the  entire  operation 
was  65°. 

Cobalt — Mercury. 
LITERATURE  I 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Kollock  states  that  the  following  conditions  are  suitable 
for  the  separation  of  these  elements:  The  solution  contained 
0.1216  g  of  mercury  as  mercuric  chloride  and  0.1  g  of  cobalt 
as  nitrate.  To  this  2  g  potassium  cyanide  were  added,  the 
total  volume  was  100  cc,  and  the  electrolysis  was  conducted 
at  65°  with  a  current  of  ND100  =  0.03  ampere  and  a  potential- 
difference  of  2.9  volts.  The  mercury  was  completely  precipi- 
tated in  5  hours. 

NICKEL. 
Nickel — Zinc. 

LITERATURE  I 
Vortmann,  Monatsch.  f.  Chemie,  14,  536  (1893). 

For  the  separation  of  these  metals  Vortmann  suggests 
a  solution  containing  from  4  to  6  grams  of  sodium  potassium 
tartrate  and  an  excess  of  sodium  hydroxide.  In  his  experi- 


246  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

ments  about  0.2  g  of  each  metal  was  present,  and  the  elec- 
trolysis was  conducted  at  ordinary  temperatures  with  a 
current  of  ND100  =  0.3-0.6  ampere.  He  states  that  under 
these  conditions  only  zinc  is  precipitated,  the  nickel  re- 
maining in  solution. 

Nickel — Manganese. 

LITERATURE  : 
Engels,  Compt.  rend.,  15,  5,  20  (1896). 

What  has  been  stated  (p.  229)  with  reference  to  the  sepa- 
ration of  iron  and  manganese  applies  equally  to  the  separa- 
tion of  nickel  and  manganese. 

Nickel — Aluminium. 

Similar  to  the  separation  of  iron  from  aluminium. 

Nickel — Uranium;   Nickel — Chromium. 

Similar  to  the  corresponding  separations  from  iron; 

Nickel — Copper. 

LITERATURE : 

Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 
Fernberger  and  Smith,  Jour.  Am.  Chem.  Soc.,  21,  1001  (1899). 

The  separation  takes  place  under  the  same  conditions  as 
the  separation  of  cobalt  from  copper. 

If  1  g  each  of  copper  sulphate  and  nickel  sulphate  are 
taken,  6  g  ammonium  oxalate  are  required.  Larger  quan- 
tities of  metal  require  correspondingly  greater  quantities  of 
ammonium  oxalate. 


NICKEL.  247 


EXPERIMENT. 

Potential-difference,   Time,  Found  ,,          , 

Volts.  hr.  m.  g  Cu.          %  Cu.  Remarks. 

1.11-1.3  3   50  0.2552      25.40  Theory  25.33*  cu. 

1.20-1.3         3-  0.2559     25.37  Acidified  with  oxalic  acid. 

1.20-1.3  330  0.2591      25.38  Acidified  with  tartaric  acid. 


1.34-1.45        3  50          0.2579      25.45     \    Acidified  with  acetic  acid      The 

(         copper  contained  nickel. 

1.20-1.6  3  50          0.2595      25.33          The  copper  contained  nickel. 

The  separation  of  these  two  metals  can  be  very  satisfac- 
torily conducted  in  a  solution  containing  free  nitric  acid 
(seep.  179). 

For  the  separation  of  nickel  and  copper  in  a  phosphate 
solution  the  following  details  are  given  by  Fernberger  and 
Smith: 

To  a  solution  containing  copper  sulphate  (  =  0.1239  g  Cu) 
and  nickel  nitrate  (  =  0.1366  g  Ni),  75  cc  of  a  solution  of  di- 
sodium  hydrogen  phosphate  (sp.  gr.  1.0358)  and  10  cc  phos- 
phoric acid  (sp.  gr.  1.347)  were"  added,  and  the  electrolysis 
was  conducted  at  66°  with  a  current  of  ND100  =  0.072  ampere 
and  a  potential-difference  of  2.45  volts,  the  total  volume  of 
the  solution  being  225  cc.  The  copper  was  completely  pre- 
cipitated in  6  hours. 


Nickel— Lead. 

The  separation  corresponds  to  the  method  given  under 
Cobalt. 

Nickel— Mercury. 

LITERATURE: 

Smith,  Am.  Chem.  Journ  ,  12,  104  (1890). 
Riidorff,  Zeit.  f.  angew  Chem.,  p.  388-(1894). 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585  (1895). 
Kollock,  Journ  Am  Chem.  Soc.,  21,  911  (1899). 

The  method  for  the  separation  of  these  two  metals  is 
similar  to  that  of  cobalt  from  mercury.     According  to  the 


"248  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

•statements  of  Smith,  the  separation  may  be  carried  out  in 
a  solution  of  the  double  cyanides.  Heidenreich,  who  de- 
termined in  the  Aachen  laboratory  the  proper  conditions 
of  experiment,  found  that  only  the  mercury  is  precipitated 
when  the  potential  at  the  electrodes  is  1.2-1.6  volts. 

EXPERIMENT. 

Used  about  1  g  nickel  ammonium  sulphate  and  3  g  potas- 
sium cyanide. 

Taken  Current-density,    Potential-difference,       T-mc  Found* 

gHgC!2.  Amperes.  Volts.  rime'  Per  Cent  Hg. 

0.3687  0.08-0.03  1.2-1.6  5|  hr.  73.65 

0.3702  0.05-0.93  1.4-1.5  overnight          73.62 

0.3000          0.05-0.03  1.4-1.5  "  73.66 

The  following  conditions  are  given  by  Kollock:  Two 
grams  of  potassium  cyanide  were  added  to  a  solution  contain- 
ing mercuric  chloride  (  =  0.1216  g  Hg)  and  nickel  nitrate 
(  =  0.1  g  Ni),  the  volume  of  the  solution  was  brought  to  125 
cc,  and  the  electrolysis  was  conducted  at  65°  with  a  current 
of  ND100=0.04  ampere  and  a  potential-difference  of  2.2  volts. 
In  four  hours  the  mercury  was  completely  precipitated. 


ZINC. 

Zinc — Manganese. 

LITERATURE  I 

Riederer,  Journ.  Am.  Chem.  Soc.,  21,  789  (1899). 
Classen,  Ausgewahlte  Methoden,  p.  386  (1902). 

The  solution  of  the  sulphates,  to  which  6  grams  of  am- 
monium oxalate  and  free  oxalic  acid  is  added,  is  electrolysed 
at  50-60°  with  a  potential-difference  between  the  electrodes 
of.  1.1  to  1.3  volts.  Under  these  conditions  the  manganese 

*  Theory,  73.80%  Hg. 


ZINC.  249 

remains  in  solution,  the  zinc  only  is  deposited.  To  deter- 
mine the  manganese  in  the  solution,  it  is  precipitated  as  Mn02 
by  adding  a  few  cubic  centimeters  of  hydrogen  peroxide 
solution  and  a  slight  excess  of  ammonia,  and  the  Mn02  after 
careful  washing  is  dissolved  in  a  mixture  of  25  cc  water,  5 
cc  acetic  acid,  and  5  cc  hydrogen  peroxide  solution.  The 
excess  of  hydrogen  peroxide  is  destroyed  with  Cr03,  and  the 
solution  is  electrolysed  as  described  on  p.  172. 

Zinc — Aluminium. 

LITERATURE  I 
Classen,  Ber.  deutsch.  chem.  Ges.,  14,  2771  (1881). 

These  elements  can  be  separated  by  the  electrolysis  of  a 
solution  containing  ammonium  oxalate  and  free  oxalic  acid, 
under  conditions  similar  to  those  given  for  the  separation 
of  zinc  from  manganese. 

Zinc — Copper. 

LITERATURE : 

Riidorff,  Zeit.  f.  angew.  Chem.,  p.  452  (1893). 
Smith  and  Wallace,  Journ.  Anal.  Chem.,  7,  183  (1893). 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585  (1895). 
Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 

For  this  separation  Smith  and  Wallace  recommend  the 
precipitation  of  the  copper  from  a  solution  containing  free 
nitric  acid.  Heidenreich,  who  determined  in  the  Aachen 
laboratory  the  proper  conditions  for  analysis,  found  that 
if  the  solution  contains  about  4  cc  free  nitric  acid  (sp.  gr. 
1 .3)  to  120  cc  of  solution,  and  a  potential-difference  between 
the  electrodes  of  1.4  volt  is  not  exceeded,  the  copper  is  pre- 
cipitated free  from  zinc.  (See  p.  179.) 


Taken 
CuSO4.5H2O 
g. 
0.4689 

ND100 
Ampere. 

0.2 

Potential  Dif- 
ference between 
Electrodes. 

1.00-1.  15  volt 

0.4728 

0.2-0.15 

1.0-1.2      ". 

0.5049 

0.15-0.2 

1.13 

0.4660 

0.5 

1.2       •  " 

250  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

EXPERIMENT. 

Used  copper  sulphate  (containing  25.29%  Cu)  to  which 
0.8  g  zinc  ammonium  sulphate  was  added. 

Time,  F™nd- 

hr.      m.  <*' 

15  30  25.25 

15  —  25.25 

18  —  25.31 

2  —  25.22 

As  illustrating  the  separation  of  these  two  elements  in 
a  solution  containing  phosphates  and  free  phosphoric  acid, 
the  following  experiment  is  described  by  Fernberger  and 
Smith: 

60  cc  of  a  solution  of  disodium  hydrogen  phosphate  (sp. 
gr.  1.0358)  and  10  cc  phosphoric  acid  (sp.  gr.  1.347)  were 
added  to  a  solution  containing  0.1239  g  of  copper  and  0.1  g 
of  zinc  as  sulphates,  the  solution  was  diluted  to  a  total  vol- 
ume of  225  cc,  warmed  to  60°,  and  electrolysed  with  a  cur- 
rent of  ND100  =  0.035  ampere  and  a  potential-difference  of 
2.5  volts.  In  5  hours  the  copper  was  completely  precipitated. 

Zinc — Bismuth. 

LITERATURE: 
Kammerer,  Journ.  Am.  Chem.  Soc.,  25,  83  (1903). 

The  following  experiment  is  described  by  Kammerer: 
To  a  solution  containing  0.15  g  bismuth  dissolved  in  1  cc 
nitric  acid  (sp.  gr.  1.42)  and  0.15  g  zinc  as  sulphate,  2  cc  of 
sulphuric  acid  (sp.  gr.  1.84)  and  0.6  g  potassium  sulphate 
were  added,  the  solution  was  diluted  to  150  cc,  and  elec- 
trolysed at  a  temperature  of  50°  with  a  current  of  ND100  =  0.02 
ampere  and  a  potential-difference  of  2.0  volts.  The  bismuth 
was  completely  precipitated  in  8  hours. 


ZINC.  251 

Zinc — Cadmium. 

LITERATURE  I 

Tver,  Bull.  Soc.  Chim.,  34,  18  (1880). 
Eliasberg,  Zeit.  f.  anal.  Chem.,  24,  550  (1885). 
Smith  and  Knerr,  Am.  Chem.  Journ.,  8,  210  (1886). 
Smith,  Am.  Chem.  Journ.,  n,  352  (1889). 
Waller,  Zeit.  f.  Elektrochem.,  4,  241  (1897-98). 

A.  Yver  has  recommended  the  use  of  a  solution  of  ace- 
tates or  sulphates  treated  with  an  excess  of  sodium  acetate 
and  a  few  drops  of  acetic  acid;  the  electrolysis  to  be  con- 
ducted with  a  warm  solution,  using  two  Daniell  cells. 

In  the  laboratory  of  the  Munich  Polytechnic  School  the  fol- 
lowing modification  of  Yver's  method  is  in  use:  To  a  sul- 
phuric acid  solution  of  the  two  metals  sodium  hydroxide 
solution  is  added  until  a  permanent  precipitate  is  formed, 
the  precipitate  is  dissolved  by^adding  the  smallest  possible 
quantity  of  dilute  sulphuric  acid, -the  solution  is  diluted  to 
about  70  cc,  and  the  cadmium  is  precipitated  with  a  current 
of  ND100  =  0.07  ampere.  When  the  greater  part  of  this  metal 
has  been  precipitated,  the  free  sulphuric  acid  is  neutralised 
with  sodium  hydroxide,  3  g  sodium  acetate  are  added,  the  so- 
lution is  warmed  to  about  45°,  and  electrolysed  with  a  current 
of  ND100  =  0.3  ampere  and  a  potential-difference  of  about 
2.4  volts. 

According  to  experiments  carried  out  in  the  author's 
laboratory  by  Waller,  the  separation  of  these  elements  can 
be  carried  out  very  satisfactorily  under  the  following  con- 
ditions :  A  solution  of  the  chlorides  from  which  the  excess  of 
hydrochloric  acid  has  been  expelled  (containing  0.1  g  of  each 
of  the  metals)  is  treated  with  8  g  potassium  oxalate  and  2g 
ammonium  oxalate,  diluted1  to  120  cc,  and  electrolysed  at 
80-85°  with  a  current  of  KD100  =  0.02  ampere.  The  cadmium 


252  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

is  precipitated.  The  zinc  remaining  in  the  solution  is  deter- 
mined by  evaporating  this  to  a  suitable  volume,  and  elec- 
trolysing at  first,  for  about  5  minutes,  with  a  current  of 
ND100  =  1.0-1.5  ampere,  the  temperature  of  the  solution 
being  about  50-60°,  and  then  as  described  under  zinc  (p.  165), 
a  tartaric  acid  solution  being  dropped  slowly  into  the  elec- 
trolyte. (Classen,  Ausgewahlte  Methoden,  p.  349.) 

Zinc — Lead. 

This  separation  can  be  conducted  in  a  solution  contain- 
ing free  nitric  acid,  the  lead  being  precipitated  as  peroxide 
(see  Lead,  p.  194).  To  determine  the  zinc,  it  is  converted 
into  sulphate  by  evaporation  with  sulphuric  acid,  and  is 
precipitated  by  the  method  given  on  page  165. 

Zinc— Silver. 

LITERATURE  : 

Smith  and  Wallace,  Journ.  Anal.  Chem.,  6,  87  (1892); 

Zeit.  f.  Elektrochem.,  2,  312  (1895-96). 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585  (1895). 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

This  separation,  according  to  Smith  and  Wallace,  can 
be  conducted  from  a  solution  containing  potassium  cyanide. 
The  experimental  conditions  ascertained  by  Heidenreich  in 
the  Aachen  laboratory  were :  temperature  60-70°,  potential- 
difference  1.9-2.0  volts,  current-density  ND100  =  0.02  to  0.08 
ampere.  Under  these  conditions  the  precipitation  of  the 
silver  takes  place  slowly. 

The  following  particulars  are  given  by  Kollock :  To  a  so- 
lution containing  0.1024  g  silver  as  nitrate  and  0.1  g  zinc 
as  sulphate,  1  g  of  potassium  cyanide  was  added  and  the 
solution  after  diluting  to  100  cc  was  electrolysed  at  60-70° 


ZINC.  253 

with  a  current  of  ND100  =  0.38  ampere  and  a  potential-differ- 
ence of  2.7  volts.  The  silver  was  completely  precipitated 
in  3  hours. 

Zinc— Mercury. 

LITERATURE ! 
Wallace  and  Smith,  Journ.  Am.  Chem.  Soc.,  18,  169  (1896); 

Zeit.  f.  Elektrochem.,  2,  312  (1896). 
Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585  (1895). 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 
Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 

Smith  and  Wallace  have  proposed  the  separation  of  these 
elements  in  a  solution  containing  potassium  cyanide.  The 
following  conditions  were  determined  by  Heidenreich  in  the 
Aachen  laboratory:  The  solution  contained  about  0.25  g  of 
mercuric  chloride  and  2-3  g  of  potassium  cyanide,  and  was 
electrolysed  with  a  current  of  ND100=0.03  to  0.08  ampere 
and  a  potential-difference  of  L65-1.75  volt.  The  mercury 
was  precipitated  free  from  zinc  in  5-14  hours. 

According  to  Kollock  the  following  conditions  proved 
satisfactory:  To  a  solution  containing  0.1158  g  of  mercury 
as  mercuric  chloride  and  0.1  g  zinc  as  zinc  sulphate,  2  g 
potassium  cyanide  was  added,  and  the  solution  having  a 
volume  of  125  cc  was  electrolysed  at  50°  with  a  current  of 
ND100  =  0.03  ampere  and  a  potential-difference  of  2.9  volts. 
The  mercury  was  completely  precipitated  in  4  hours. 

For  the  separation  of  mercury  and  zinc  from  a  solution 
containing  phosphates  and  free  phosphoric  acid,  we  are  in- 
debted to  Fernberger  and  Smith  for  the  following  particulars: 

60  cc  disodium  hydrogen  phosphate  (sp.  gr.  1.038)  and 
10  cc  phosphoric  acid  (sp.  gr.  1.347)  were  added  to  a  solution 
containing  mercuric  chloride  (  =  0.1159  g  Hg)  and  zinc  sul- 
phate (  =  0.1010  g  zinc).  The  volume  of  the  final  solution 
was  175  cc,  and  this  solution  was  electrolysed  at  60°  with  a 


254  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

current  of  ND100  =  0.01  and  a  potential-difference  of  1.5  volt. 
The  mercury  was  precipitated  in  4  to  5  hours. 

The  separation  of  these  two  metals  can  also  be  conducted 
in  solutions  containing  free  acid  (p.  204). 

MANGANESE. 
Manganese — Copper. 

LITERATURE  I 

Fernberger  and  Smith,  Journ.  Am.  Chem.  Soc.,  21,  1001  (1899). 
Classen,  Ausgewahlte  Methoden,  p.  386  (1902). 

The  separation  of  manganese  and  copper  can  be  con- 
ducted under  conditions  exactly  similar  to  those  given  for 
the  separation  of  manganese  and  zinc  (p.  248). 

According  to  Fernberger  and  Smith  this  separation  may 
be  conducted  in  a  solution  containing  phosphates  and  free 
phosphoric  acid  under  the  following  conditions : 

To  a  solution  containing  about  0.1236  g  of  each  of  the 
metals,  60  cc  of  a  solution  of  disodium  hydrogen  phosphate 
(sp.  gr.  1.038)  and  10  cc  phosphoric  acid  (sp.  gr.  1.347) 
were  added,  the  solution  was  diluted  to  225  cc,  and  elec- 
trolysed at  56°  with  a  current  of  ND100  =  0.05  ampere  and  a 
potential-difference  of  2.5  volts.  The  copper  was  precipitated 
in  about  6  hours. 

Manganese — Cadmium. 

This  separation  may  be  conducted  under  the  same  con- 
ditions as  those  given  for  the  separation  of  manganese  and 
zinc  (p.  248). 

Manganese —  Bismuth. 

LITERATURE  I 
Kammerer,  Journ.  Am.  Chem.  Soc.,  25,  83  (1903). 

This  separation,  according  to  Kammerer,  can  be  carried 
out  under  the  following  conditions : 


MANGANESE.  255 

To  a  solution  containing  0.15  g  of  bismuth  dissolved  in 
1  cc  nitric  acid  (sp.  gr.  1.42)  and  manganous  sulphate 
(  =  0.15  g  Mn),  3  cc  sulphuric  acid  (sp.  gr.  1.84)  and  0.5  g 
potassium  sulphate  were  added,  the  solution  was  diluted 
to  150  cc,  and  was  electrolysed  at  45°  with  a  current  of  ND100 
=  0.025  ampere  and  a  potential-difference  of  2.0  volts.  The 
bismuth  was  completely  precipitated  in  9  hours,  and  although 
some  manganese  dioxide  separated  at  the  anode  it  was  not 
found  to  contain  any  bismuth. 

Manganese — Lead. 

LITERATURE  I 

Neumann,  Chem.  Ztg.,  20,  No.  39  (1896). 

Hansen,  Chem.  Ztg.,  25,  393  (1901). 

Classen,  Ausgewahlte  Methoden,  p.  386  (1902). 

The  separation  depends  upon  the  fact  that  in  a  solution 
containing  more  than  3  or  4  per  cent,  of  free  nitric  acid  no 
manganese  dioxide  is  precipitated  on  electrolysis,  while  under 
the  same  conditions  the  lead  is  precipitated  as  peroxide  on 
the  anode.  Fairly  accurate  results  can  be  obtained  when 
the  solution  does  not  contain  more  than  0.03  g  Mn  in  150  cc, 
and  the  electrolysis  is  conducted  at  40-70°  with  a  current  of 
ND100  =  1.5-2  amperes  and  a  potential-difference  of  2.5  volts. 
If  the  solution  contains  a  greater  proportion  of  manganese, 
or  if  the  electrolysis  is  continued  too  long  after  all  the  lead 
peroxide  has  separated,  a  flocculent  precipitate  of  MnO2  will 
appear  in  the  solution,  and  the  precipitated  lead  peroxide 
will  contain  manganese  dioxide  (Classen). 

The  other  methods  proposed  for  the  separation  of  these 
two  elements  are  of  questionable  accuracy.  For  further 
information  the  original  articles  should  be  consulted. 


256  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

COPPER. 

Copper — Cadmium. 
LITERATURE ! 

Smith,  Am.  Chem.  Journ.,  12,  329  (1891). 
Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  i,  299  (1892). 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  122  (1893). 
Smith  and  Wallace,  Journ.  Anal.  Chem.,  7,  253  (1893). 
Heidenreieh,  Ber.  deutsch.  chem.  Ges.,  28,  1585  (1895). 
Wallace  and  Smith,  Journ.  Am.  Chem.  Soc.,  19,  870  (1897). 
Rimbach,  Zeit.  f.  anal.  Chem.,  37,  284  (1898). 

According  to  the  statements  of  Freudenberg,  the  metals 
can  be  separately  precipitated  from  a  solution  containing 
10-20  cc  dilute  sulphuric  acid.  When  such  a  solution  is 
electrolysed  with  a  difference  of  potential  between  the  elec- 
trodes of  not  more  than  2  volts  only  the  copper  is  deposited. 

Heidenreieh  tested  this  method  in  the  Aachen  laboratory 
and  found  that  the  separation  is  best  conducted  with  a  dif- 
ference of  potential  not  exceeding  1.85  volt. 

EXPERIMENT. 

The  volume  of  the  liquid  was  120  cc  and  it  contained  15 
cc  dilute  sulphuric  acid  (sp.  gr.  1.09).  The  copper  sulphate 
used  contained  25.29%  Cu. 

m  i  Current-density          Potential-         rp-  Found 

r,<sn  m-nrSwin  RTT  n  NDi00,  difference,        Vme>         Cu- 

CuS04.5H20,  CdS04.8H20.  Amperes.  Volts.  hrs"  % 

0.7078  0.40  0.07-0.05          1.7-1.76        24        25.27 

The  time  required  for  the  precipitation  of  the  copper 
can  be  considerably  shortened  by  warming. 

Experiments  in  which  it  was  attempted  to  replace  the 
sulphuric  acid  with  nitric  acid  yielded  no  satisfactory  results. 

To  determine  the  cadmium  remaining  in  the  solution 
from  which  the  copper  has  been  removed,  the  solution  is 


COPPER.  257 

neutralised  with  sodium  hydroxide,  and  the  cadmium  is 
precipitated  from  a  solution  containing  ammonium  oxalate 
or  potassium  cyanide,  as  described  under  Cadmium. 

The  separation  of  these  two  metals  can  be  carried  out 
in  a  shorter  time  in  a  solution  containing  free  nitric  acid. 
The  salts  are  dissolved  in  water,  5  cc  of  nitric  acid  (sp.  gr. 
1.21)  are  added,  the  whole  is  diluted  to  150  cc,  and  elec- 
trolysed at  ordinary  temperatures  with  a  current  of  ND100  = 
1.0  ampere  and  a  potential-difference  of  2.8  to  2.9  volts. 
The  copper  is  precipitated.  To  determine  the  cadmium  in 
the  residual  solution  this  is  evaporated  with  an  excess  of 
sulphuric  acid,  to  convert  the  cadmium  into  sulphate,  and 
the  cadmium  is  precipitated  from  a  solution  containing  am- 
monium oxalate.  (Classen,  Ausgewahlte  Methoden,  p.  115.) 

According  to  Smith,  the  separation  may  also  be  con- 
ducted in  a  solution  containing  phosphates  and  free  phos- 
phoric acid.  With  this  end  in"  view  20  cc  disodium  hydro- 
gen phosphate  solution  (sp.  gr.  1.035)  and  10  cc  phosphoric 
acid  (sp.  gr.  1.35)  are  added  to  the  solution,  which  is  diluted 
to  125  cc  and  electrolysed  at  60°  with  a  current  of  ND100  = 
0.08  ampere  and  a  potential-difference  of  2.5  volts.  .  The  cop- 
per is  deposited  in  about  3  hours. 

Copper — Lead. 

LITERATURE: 

May,  Am.  Joura.  Science,  [3]  6,  255  (1873). 
Nissenson,  Zeit.  f.  angew.  Chem.,   pp.  452,  646  (1893). 
Classen,  Ber.  deutsch.  chem.  Ges.,  27,  2060  (1894). 

To  separate  copper  from  lead,  20  cc  of  nitric  acid  (sp. 
gr.  1.35)  are  added  to  the  solution,  which  is  then  diluted  to 
75  cc,  warmed,  and  electrolysed  with  a  current  of  ND100  = 
1.5  to  1.7  ampere,  a  roughened  dish  serving  as  anode.  At 
the  end  of  one  hour  the  greater  part  (98-99%  when  not  more 


:258          QUANTITATIVE    ANALYSIS   BY   ELECTROLYSIS. 

than  0.5  g  is  present)  of  the  lead  will  have  separated  as  per- 
oxide, and  the  current  is  then  interrupted;  no  trace  of  cop- 
per having  as  yet  appeared  on  the  cathode.  The  liquid 
should  then  be  transferred  to  a  second  weighed  dish,  and  the 
lead  peroxide  on  the  first  washed  with  water,  dried,  and 
weighed.  The  washings  from  the  lead  peroxide  are  added 
to  the  solution  in  the  second  dish,  which  is  treated  with  am- 
monia until  the  well-known  deep-blue  color  appears,  and 
then  5  cc  of  nitric  acid  are  added.  The  second  platinum 
dish  is  connected  as  cathode,  and  the  perforated  platinum 
dish-electrode  described  on  page  112  is  used  as  anode.  The 
surface  of  the  anode  should  be  roughened,  and  its  weight 
should  be  accurately  determined  before  the  experiment. 
The  solution  is  diluted  to  120-150  cc,  allowed  to  cool,  and 
-electrolysed  with  a  current  of  ND100  =  1.0-1.2  ampere.  The 
copper  will  be  precipitated  on  the  cathode  and  the  remainder 
of  the  lead  as  peroxide  on  the  anode.  The  time  required, 
when  0.25  g  of  copper  is  present,  is  about  4  hours. 

This  method,  which  is  of  great  value  in  technical  work 
is  not  only  rapid  (4-5  hours  as  compared  with  14  hours  or 
more),  but  allows  of  the  complete  precipitation  of  both 
metals,  irrespective  of  the  relative  quantities  present  (see 
page  292). 

Copper — Silver. 
LITERATURE  I 

Luckow,  Zeit.  f.  anal.  Chem.,  19,  15  (1880). 

Smith  and  Frankel,  Am.  Chem.  Journ.,  12,  104  (1891). 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  107  (1893). 

Smith  and  Wallace,  Zeit.  f.  Elektrochem.,  2,  312  (1895). 

Heidenreich,  Ber.  deutsch.  chem.  Ges.,  28,  1585  (1895). 

Kiister  and  Steinwehr,  Zeit.  f.  Elektrochem.,  4,  451  (1897-98). 

Revay,  Zeit.  f.  Elektrochem.,  4,  313  (1897-98). 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Brunck,  Ber.  deutsch.  chem.  Ges.,  34,  1604  (1901). 

Tulweiler  and  Smith,  Journ.  Am.  Chem.  Soc.,  23,  582  (1901). 


COPPER.  259 

According  to  Freudenberg,  this  separation  can  be  car- 
ried out  in  a  solution  containing  2  to  3  cc  nitric  acid  (sp.  gr. 
1.2),  by  conducting  the  electrolysis  with  a  potential-differ- 
ence of  from  1.3  to  1.4  volt.  Under  these  conditions  the 
silver  is  precipitated  free  from  copper.  The  copper  remain- 
ing in  the  solution  can  be  afterwards  precipitated  by  in- 
creasing the  difference  of  potential  to  2  to  3  volts.  The 
time  required  for  the  analysis  is  reduced  about  one-half  by 
warming  the  solution. 

According  to  Smith  and  Frankel,  these  two  metals  can 
be  separated  in  a  solution  containing  the  double  cyanides 
with  potassium.  4.5  g  of  potassium  cyanide  are  added  to  a 
solution  containing  about  0.4  g  of  the  metals.  The  solution 
is  diluted  to  120  cc  and  electrolysed.  If  the  electrolyte 
is  warmed  to  65-75°,  the  precipitation  of  the  silver  is  greatly 
hastened.  M.  Heidenreich  tested  this  method  in  the  Aachen 
laboratory,  and  determined  the*  following  conditions  for 
analysis. 

EXPERIMENT. 

Used  silver  nitrate,  containing  63.42%  silver,  and  about 
0.7  g  of  copper  sulphate  in  each  experiment. 

Time,  F°»nd 

A|' 
63.34 
63.43 

30  63.40 

(warmed)     63.27 
63.33 

According  to  Freudenberg  the  maximum  difference  of 
potential  used  must  not  exceed  2.3  volts,  or  both  metals  will 
be  precipitated.  The  copper  remaining  in  the  solution  after 


Taken 
AgNO3,       KCN, 
g.                g- 

Current- 
density, 
Amperes. 

Potential- 
difference, 
Volts. 

T 
hr, 

0.2379 

2 

0.07-0.03 

1.0-1.2 

8 

0.2303 

2 

0.04 

1.0-1.3 

8 

0.3099 

2 

0.03 

1.0-1.4 

6 

0.3327 

2 

0.09 

1.2-1.3 

4 

0.6037 

6 

0.19-0.08 

1.2-1.3 

6 

260  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

the  silver  has  been  removed  can  be  deposited  by  electrolysing 
with  a  higher  difference  of  potential. 

Kollock  describes  the  following  experiment:  Two  grams 
of  potassium  cyanide  were  added  to  a  solution  containing 
0.1024  g  silver  and  0.098  g  copper,  both  present  as  nitrate, 
the  solution  was  diluted  to  125  cc,  warmed  to  65°,  and  elec- 
trolysed with  a  current  of  ND100  =  0.03-0. 05  ampere  and  a 
potential-difference  of  1.1-1.6  volt.  The  silver  was  com- 
pletely precipitated  in  3-7  hours. 

Fulweiler  and  Smith  recommend  that  the  solution,  after 
the  separation  of  the  silver,  be  diluted  to  500  cc  and  elec- 
trolysed with  a  higher  potential-difference  to  precipitate 
the  copper. 


Copper — Mercury. 
LITERATURE  I 

Smith,  Journ.  Anal.  Chem.,  3,  254  (1889) ; 

Am.  Chem.  Journ.,  u,  104,  264  (1889); 

Journ.  Anal.  Chem.,  5,  489  (1891). 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  113  (1893). 
Smith,  Journ.  Am.  Chem.  Soc.,  16,  42  (1894). 
Revay,  Zeit.  f.  Elektrochem.,  4,  313  (1897-98). 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 
Spare  and  Smith,  Journ.  Am.  Chem.  Soc.,  23,  597  (1901). 

According  to  Edgar  F.  Smith,  the  separation  can  be  con- 
ducted in  a  solution  containing  the  double  cyanides.  The 
temperature  should  be  about  65°.  Under  the  ordinary 
conditions  of  concentration  (150  cc)  about  2  grams  of  po- 
tassium cyanide  are  added,  and  the  solution  is  electrolysed 
with  a  current  of  ND100  =  0.06-0.08  ampere.  The  electroly- 
sis requires  about  4  hours  for  every  0.2  g  of  the  metals  pres- 
ent. The  mercury  is  deposited,  the  copper  remaining  in  the 
solution. 


COPPER.  261 

Freudenberg  found  that  in  the  presence  of  2-4  g  potas- 
sium cyanide  when  the  difference  of  potential  between  the 
•electrodes  is  maintained  at  2.5  volts,  the  mercury  separates 
brilliantly  white  and  entirely  free  from  copper. 

The  copper  remaining  in  the  solution  after  the  mercury 
has  been  separated  is  precipitated  by  electrolysing  the  solu- 
tion warmed  to  60°  with  a  potential-difference  of  4.2  volts 
between  the  electrodes. 

The  two  following  experiments  are  described  by  Spare 
and  Smith: 

1.  To  a  solution   containing   0.1211    g  of  mercury  and 
0.1520  g  of  copper,  2.5  g  of  potassium  cyanide  were  added. 
The  solution  after  dilution  to  125  cc  was  electrolysed  at  63° 
with  a  current  of  ND100  =  0.03-0.05  ampere  and  a  potential- 
difference  of  1.2-1 . 9  volts.     In  from  2J  to  4  hours  the  mercury 
was  completely  precipitated. 

2.  To  a  solution  containing  ~  0.0453  g  of  mercury  and 
0.5115  g  of  copper,  5.5-7.5  g  of  potassium  cyanide  were  added. 
The  solution  was  diluted  to  135  cc.  and  was  electrolysed  at 
60°  with  a  current  of  ND100  =  0.01-0.03  ampere  and  a  poten- 
tial-difference   of    1.1-1.5    volts.     The    mercury    was    com- 
pletely precipitated  in  2^  to  3J  hours. 

Copper — Arsenic;   Copper — Antimony. 

LITERATURE ! 

Drossbach,  Chem.  Zeitung,  16,  819  (1892). 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97  (1893). 
Oettel,  Chem.  Zeitung,  18,  879  (1894). 
Schmucker,  Zeit.  f.  anorg.  Chem.,  5,  199  (1894). 
Hollard,  Compt.  rend.,  123,  1063  (1896). 
Head,  Berg-  u.  Huttenm.  Ztg.,  57,  139  (1898). 
Lucas,  Bull.  Soc.  Chim.,  19,  817  (1898). 

Freudenberg  has  shown  that  the  separation  of  copper 
and  arsenic  can  be  satisfactorilv  conducted  in  a  solution 


262  QUANTITATIVE  ANALYSIS    BY    ELECTROLYSIS. 

containing  10-20  cc  dilute  sulphuric  acid  if  the  difference  of 
potential  between  the  electrodes  is  not  allowed  to  exceed 
1.9  volts.  It  is  immaterial  whether  the  arsenic  is  present  as 
arsenious  or  arsenic  acid.  A  second  method  suggested  by 
the  same  author  is  the  following :  Ammonia  is  added  to  a 
solution  containing  the  metals  in  the  form  of  higher  oxides, 
until  there  is  an  excess  of  about  30  cc  of  10%  ammonia 
present.  The  electrolysis  is  conducted  with  a  difference  of 
potential  between  the  electrodes  of  1.9  volts,  and  is  continued 
until  the  solution  is  completely  decolorised,  requiring  gen- 
erally 6-8  hours.  This  method  is  not  suitable  for  the  sepa- 
ration of  copper  and  antimony. 

Copper  can  be  separated  from  arsenic  and  small  quan- 
tities of  antimony  by  the  electrolysis  of  a  solution  containing 
ammonium  nitrate  and  free  ammonia  (Rudorff's  method, 
p.  182)  with  a  current  of  ND100=  0.07-0.27.  The  proportion 
of  ammonia  present  must  not  be  too  great  or  too  small.  If 
the  latter,  a  brown  deposit  forms  on  the  anode,  which  becomes 
detached  and  produces  black  spots  on  the  precipitated  cop- 
per, causing  the  weight  of  this  to  improperly  increase.  Large 
quantities  of  ammonium  nitrate  have  a  beneficial  influence. 
According  to  Oettel,  the  concentration  of  the  solution  must 
not  exceed  0.8  g  of  copper  in  100  cc  (Classen,  Ausgewahlte 
Methoden,  p.  80). 

According  to  Head,  a  copper  solution  containing  arsenic 
and  antimony  can  be  freed  from  the  two  latter  so  that  on 
electrolysis  only  pure  copper  will  be  precipitated : 

The  solution  is  evaporated  to  complete  dryness  and  10  cc 
of  bromine,  in  which  2  g  of  sulphur  has  been  dissolved,  is 
added.  The  mixture  is  evaporated  to  a  pa,sty  mass,  20  cc 
of  pure  bromine  is  then  added,  and  this  mixture  is  heated 
until  all  of  the  antimony  has  been  driven  off  as  a  white  vapor 
and  the  residue  is  dry  and  light  gray  in  color.  The  dish 


BISMUTH.  263 

must  be  covered  as  long  as  there  is  chance  of  loss  from  spat- 
tering, and  the  copper  bromide  must  not  be  too  strongly 
heated,  since  in  this  case  loss  would  occur.  The  residue 
contains  only  traces  of  antimony,  and  is  suitable  for  elec- 
trolysis after  it  has  been  converted  into  sulphate. 

BISMUTH. 
Bismuth —  Cadmium. 

LITERATURE  I 
Kammerer,  Journ.  Am.  Chem.  Soc.,  25,  83  (1903). 

The  following  experiment  is  described  by  Kammerer: 
0.15  g  of  bismuth  was  dissolved  in  1  cc  nitric  acid  (sp.  gr. 
1.42)  and  to  this  was  added  0.15  g  of  cadmium  oxide  dis- 
solved in  2  cc  of  sulphuric  acid  (sp.  gr.  1.84).  To  the  above 
mixture  1  g  of  potassium  sulphate  was  added,  the  solution 
was  diluted  to  150  cc,  and  electfolysed  at  50°  with  a  current 
of  ND100  =  0.025  ampere  and  a  potential-difference  of  2.0 
volts.  The  cadmium  was  completely  precipitated  in  8  hours. 

Bismuth —  Uranium. 

LITERATURE  I 
Kammerer,  Journ.  Am.  Chem.  Soc.,  25,  83  (1903). 

Kammerer  describes  this  separation  as  follows:  To  a 
solution  containing  0.15  g  of  bismuth  dissolved  in  1  cc 
nitric  acid  (sp.  gr.  1.43)  and  uranium  sulphate  equivalent 
to  0.1  g  Ur,  1  g  of  potassium  sulphate  and  2  cc  of  sulphuric 
acid  (sp.  gr.  1.84)  were  added,  and  after  diluting  to. a  volume 
of  150  cc  the  bismuth  was  precipitated  with  a  current  of 
ND100^=  0.025  ampere  and  a  potential-difference  of  2  volts. 
The  temperature  of  "the  solution  throughout  the  operation 
was  45°  and  the  time  required  was  8  hours. 


264  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

CADMIUM. 
Cadmium — Lead. 

The  method  is  the  same  as  that  in  the  separation  of  lead 
from  copper,  except  that  the  precipitation  of  the  lead  is  con- 
ducted in  one  operation. 

To  determine  the  cadmium  in  the  solution  from  which 
the  lead  has  been  removed,  this  is  converted  into  sulphate 
by  evaporation  with  sulphuric  acid,  and  is  precipitated  by 
electrolysis  by  one  of  the  methods  described  on  page  188. 

Cadmium —  Silver. 

LITERATURE  : 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

The  following  experiment  is  described  by  Kollock:  Two 
grams  of  potassium  cyanide  were  added  to  a  solution  contain- 
ing silver  nitrate  (  =  0.1024  g  Ag)  and  cadmium  sulphate 
(  =  0.168  g  Cd),  the  mixture  was  diluted  to  125  cc,  and 
was  electrolysed  at  65°  with  a  current  of  ND100  =  0.02  am- 
pere and  a  potential-difference  of  2.15  volts.  At  the  end 
of  5  hours  the  silver  was  completely  precipitated. 

Cadmium — Mercury. 

LITERATURE: 

Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97  (1893). 

Smith  and  Wallace,  Journ.  Am.  Chem.  Soc.,  17,  612  (1895). 

Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

According  to  Freudenberg,  these  metals  are  best  sepa- 
rated in  a  solution  containing  0.5-1.0  g  potassium  cyanide. 
When  the  electrolysis  is  conducted  with  a  potential-differ- 
ence between  the  electrodes  of  from  1.8  to  1.9  volts,  only  the 
mercury  is  precipitated.  After  the  separation  of  the  mer- 


LEAD.  265 

cury  the  cadmium  is  precipitated  from  the   solution  by  a 
current  having  a  higher  difference  of  potential. 

Kollock  describes  an  experiment  in  which  2.5  g  of  potas- 
sium cyanide  were  added  to  a  solution  containing  0.1182  g 
of  mercury  as  mercuric  chloride  and  0.2  g  of  cadmium  as 
sulphate,  and  the  solution  having  a  volume  of  125  cc  was 
electrolysed  at  65°  with  a  current  of  ND100  =  0.018  ampere 
and  a  potential-difference  of  1.7  volt.  The  mercury  was 
completely  precipitated  in  7  hours. 

LEAD. 
Lead— Silver. 
LITERATURE: 

Luckow,  Zeit.  f.  angew.  Chem.,  p.  345  (1890). 

Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  4,  267  (1893). 

This  separation  can  be  carried  out  like  that  of  lead  from 
copper  (see  page  257).  To  determine  the  silver  in  the  residual 
solution  it  is  evaporated  down  to  a  smaller  volume  on  the 
water-bath,  and  the  silver  is  precipitated  according  to  the 
directions  given  on  page  200. 

Lead — Mercury. 

LITERATURE  I 

Smith  and  Moyer,  Zeit.  f.  anorg.  Chem.,  4,  267  (1893). 
Heidenreich,  Ber.  deustch.  chem.  Ges.,  28,  1585  (1895). 

The  method  corresponds  to  that  used  for  the  separation 
of  copper  and  lead.  Smith  and  Moyer  have  suggested  the 
determination  of  the  lead  and  mercury  at  the  same  time, 
the  lead  being  precipitated  as  peroxide  on  the  anode,  the 
mercury  as  metal  on  the  cathode.  The  conditions  suitable 
for  this  separation,  according  to  Heidenreich,  are  that  20-30 


266  QUANTITATIVE    ANALYSIS   BY   ELECTROLYSIS. 

cc  nitric  acid  (sp.  gr.  1.3-1.4)  must  be  present  in  every  120 
cc  of  the  solution,  which  should  be  electrolysed  with  a  cur- 
rent of  ND100  =  0.2-0.5  ampere. 

According  to  Smith  (Electro-Chemical  Analysis,  1902, 
p.  152),  the  solution  should  have  a  volume  of  175  cc  and 
should  contain  20-30  cc  of  nitric  acid  (sp.  gr.  1.3),  and  the 
electrolysis  should  be  conducted  at  30°  with  a  current  of 
NDJOO  =  0.13  to  0.18  ampere  and  a  potential-difference  of 
2  volts,  for  4  hours.  A  platinum  dish  should  be  used  as  anode. 

SILVER. 

Silver — Antimony. 
LITERATURE : 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97  (1893). 

If  the  antimony  is  present  as  pentoxide,  the  separation 
can  be  carried  out  in  an  ammoniacal  solution  to  which  sev- 
eral grams  of  ammonium  sulphate  have  been  added.  In  this 
case  the  difference  of  potential  at  the  electrodes  should 
be  so  regulated  that  it  is  between  1.2-1.3  volts.  Since  under 
these  conditions  the  current-density  at  the  cathode  is  so  low 
that  the  deposited  silver  does  not  adhere  firmly  to  the  elec-, 
trode,  it  is  better  to  add  to  the  solution  1  g  of  potassium 
cyanide  for  0.1  g  of  metal  present  and  conduct  the  elec- 
trolysis with  a  potential-difference  of  2.3-2.4  volts. 

Silver — Arsenic. 
LITERATURE  I 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97  (1893). 

According  to  Freudenberg,  this  separation  can  be  carried 
out  in  the  same  manner  as  the  separation  of  silver  from 
antimony. 


MERCURY.  267 


MERCURY. 

Mercury — Antimony. 
» 

LITERATURE : 

Freudenberg,  Zeit.  f.  phys.  Chem.,  21,  97  (1893). 

The  antimony  must  be  added  in  the  form  of  a  pentavalent 
salt,  since  otherwise  a  reduction  of  the  mercuric  salt  present 
will  occur.  A  mixture  of  the  chlorides  of  the  two  metals 
can  be  brought  into  solution  by  the  addition  of  0.5-1  g  of  tar- 
taric  acid.  The  solution  is  diluted  with  water,  neutralised 
with  ammonia,  and  about  20  cc  of  a  ten-per-cent.  solution 
of  ammonia  added  until  the  solution  is  perfectly  clear.  The 
electrolysis  is  conducted  with  a  difference  of  potential  of  1.6 
to  1.7  volts.  After  the  mercury  has  been  deposited,  the  solu- 
tion is  acidified  with  hydrochloric  acid  and  treated  with  hy- 
drogen sulphide.  The  precipitated  antimony  sulphide  can 
be  determined  gravimetrically  or  by  electrolysis  as  de- 
scribed on  p:  210. 

Mercury — Arsenic . 

LITERATURE  I 
Freudenberg,  Zeit.  f.  phys.  Chem.,  12,  97  (1893). 

According  to  Freudenberg,  this  separation  can  be  carried 
out  in  a  solution  containing  nitric  acid  (see  page  204)  from 
which  the  mercury  is  precipitated  by  a  difference  of  poten- 
tial between  the  electrodes  of  1.7-1.8  volts. 


268  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

ANTIMONY. 
Antimony — Tin. 

LITERATURE  I 

Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2245  (1884); 

ibid.,  18,  1110  (1885);  ibid.,  28,  2060  (1895). 
Waller,  Zeit.  f.  Elektrochemie,  4,  241  (1897-98). 
Ost  and  Klapproth,  Zeit.  f.  angew.  Chem.,  p.  827  (1900); 
Zeit.  f.  Elektrochem.,  7,  376  (1900). 

The  separation  of  antimony  from  tin  by  the  ordinary 
gravimetric  methods,  as  is  well  known,  is  difficult  and  gives 
in  most  cases  uncertain  results.  The  electrolytic  separa- 
tion, however,  can  be  conducted  with  ease  and  accuracy. 
Antimony,  in  the  presence  of  tin,  can  be  completely  precipi- 
tated from  a  concentrated  solution  of  sodium  sulphide  to 
which  the  proper  amount  of  sodium  hydroxide  has  been 
added. 

The  crystallised  sodium  monosulphide  of  commerce, 
aside  from  the  fact  that  its  purity  is  otherwise  uncertain,  is 
not  pure  monosulphide,  but  a  mixture  of  several  sulphides 
with  varying  amounts  of  sodium  hydroxide.  This  explains 
the  large  per  cent,  of  alumina  which  it  often  contains. 
If,  therefore,  commercial  sodium  sulphide  is  used,  it  must 
first  be  dissolved  in  water,  and  the  solution,  with  exclusion 
of  air,  completely  saturated  with  pure  hydrogen  sulphide  gas. 
It  is  then  filtered  from  the  precipitated  impurities  and  con- 
centrated by  evaporation  in  a  platinum  or  porcelain  dish. 
The  further  treatment  is  given  in  the  chapter  on  reagents 
(p.  294).  As  the  purity  of  the  sodium  sulphide  solution 
used  is  of  great  importance  to  the  success  of  the  process,  it 
is  desirable  to  prepare  the  solution  as  directed  in  the  chapter 
referred  to. 


ANTIMONY.  269 

The  process  of  separation  is  as  follows :  A  mixture  of  the 
pure  sulphides,*  or  the  residue  obtained  by  evaporating  a 
solution  of  the  two  metals  to  dryness,  is  treated  with  about 
80  cc  of  a  saturated  solution  of  sodium  sulphide  (saturated 
at  ordinary  temperature)  and  enough  concentrated  solution  of 
pure  sodium  hydroxide  to  furnish  an  excess  of  1-2  g  NaOH. 
If  solution  does  not  take  place  at  once,  it  is  hastened  by  warm- 
ing over  a  low  flame.  The  preparation  of  this  solution  is 
best  conducted  in  the  platinum  dish  in  which  the  electrolysis 
is  to  be  carried  out. 

The  electrolysis  can  be  conducted  at  a  temperature  of 
50-60°  with  a  difference  of  potential  between  the  electrodes 
of  not  more  than  0.7  volt.  Under  these  conditions  the  cur- 
rent-density will  be  about  ND100  =  0.5  ampere,  and  the  pre- 
cipitation of  the  antimony  will  be  complete  in  about  two 
hours.  The  separation  can  also  be  conducted  in  the  cold 
with  a  current  of  ND100=0.2-Or4  ampere  and  a  potential- 
difference  of  0.5-0.7  volt,  in  which  case  the  time  required 
will  be  about  fourteen  hours.  To  obtain  satisfactory  results 
the  inner  surface  of  the  platinum  dish  (cathode)  should  be 
roughened. 

When  the  electrolysis  begins,  the  whole  inner  surface  of 
the  dish  which  is  in  contact  with  the  solution  becomes 
quickly  covered  with  a  dark  coating  of  antimony,  which  soon 
takes  on  a  brilliant  metallic  appearance. 

In  the  earlier  part  of  the  process  the  entire  solution 
appears  to  be  filled  with  small  gas-bubbles,  which  rise  slowly, 
break  at  the  surface,  and  project  minute  portions  of  the 
solution  against  the  lower  side  of  the  w^tch-glass  covering 
the  liquid.  After  about  two  hours  the  disengagement  of 

*  A  solution  in  sodium  sulphide  of  the  metal  sulphides  and  sulphur 
should  be  treated  like  a  solution  of  polysulphides  (p.  211). 


270  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

gas  ceases  and  the  solution  becomes  clear.  To  avoid  loss, 
it  is  best,  at  this  time,  to  wash  repeatedly  the  under  surface 
of  the  cover-glass  with  a  drop  of  water,  which  is  finally 
allowed  to  run  down  the  positive  electrode.  When  the  pre- 
cipitation of  the  antimony  is  complete,  the  deposited  metal 
is  washed  without  interrupting  the  current,  and  is  then 
treated  according  to  the  directions  given  on  page  211. 

Since  tin  cannot  be  precipitated  by  electrolysis  from  a 
solution  containing  sodium  sulphide,  but  can  readily  be  pre- 
cipitated from  one  containing  ammonium  sulphide,  the  so- 
dium sulphide,  after  the  separation  of  antimony,  must  be 
converted  into  ammonium  sulphide  according  to  the  direc- 
tions given  on  p.  215. 

If  tin  and  antimony  are  to  be  determined  in  a  solution 
containing  the  yellow  polysulphides  of  the  alkalies,  the  solu- 
tion must  be  decolorised  with  ammoniacal  hydrogen  per- 
oxide (see  Antimony,  p.  211),  and  then  evaporated  nearly  to 
drjoiess;  about  80  cc  of  a  saturated  solution  of  sodium  sul- 
phide and  the  necessary  amount  of  sodium  hydroxide  are  then 
added  and  the  process  is  carried  out  as  described  above. 

In  the  following  experiments  antimony  was  precipitated 
from  both  warm  and  cold  solutions  containing  tin. 


EXPERIMENTS. 

Used  about  1  g  antimony  potassium  tartrate,  an  equal 
weight  of  ammonium  stannic  chloride,  80  cc  sodium  sulphide 
solution,  and  about  2  g  sodium  hydroxide. 


Current- 
density, 
Amperes. 

Potential- 
difference, 

Volts. 

Temp. 

Time, 
hrs. 

Taken 
Antimony, 
g. 

Found 
Antimony, 
g- 

1.4-1.5 

0.8-0.9 

57-67° 

2 

0.3780 

0.3790 

1.5-1.6 

0.8-0.9 

58-60° 

2 

0.3780 

0.3787 

0.4-0.2 

0.7-0.55 

24-30° 

15 

0.3780 

0.3775 

ANTIMONY.  271 

The  precipitated  antimony  was  gray  in  color  and  metallic 
in  appearance,  and  it  contained  no  tin. 

Antimony — Arsenic. 

LITERATURE : 

Classen  and  Ludwig,  Ber.  deutsch.  chem.  Ges.,  19,  323  (1886). 
Classen,  Zeit.  f.  Elektrochem.,  i,  291  (1894-95). 

In  an  alkaline  solution  arsenious  acid  is  oxidised  to  arsenic 
acid  by  the  action  of  the  electric  current.  If,  however,  a 
solution  containing  both  antimony  and  arsenious  acid  is 
electrolysed,  a  mixture  of  antimony  and  arsenic  is  deposited. 
The  action  is  different  if  the  arsenic  is  present  in  the  solution 
as  arsenic  acid ;  in  the  presence  of  a  free  alkali,  the  antimony 
alone  is  precipitated  from  a  concentrated  sodium  sulphide 
solution. 

The  arsenic  if  present  as  arsenious  acid  must  be  oxidised 
to  arsenic  acid  before  it  can  be  separated  from  antimony. 
Nitric  acid  or  aqua  regia  should  be  added  to  the  solution,  the 
acid  completely  expelled  by  evaporating  to  dryness  on  a  water- 
bath,  the  residue  treated  with  80  cc  of  a  cold  saturated  solu- 
tion of  sodium  sulphide,  a  concentrated  solution  of  sodium 
hydroxide  (containing  1-2  g  NaOH)  added,  and  this  solution 
electrolysed.  The  operation  is  conducted  under  the  same 
conditions  as  in  the  separation  of  antimony  from  tin,  and  the 
electrolyte  can  either  be  warm  or  at  the  ordinary  tempera- 
ture. 

If  the  solution  containing  the  arsenic  and  antimony  also 
contains  polysulphides,  the  latter  should  be  destroyed  as 
described  on  p.  211. 

To  determine  the  arsenic,  the  antimony-free  solution  is 
acidified  with  dilute  sulphuric  acid,  heated  on  the  water-bath 
to  expel  the  hydrogen  sulphide,  filtered,  and  the  precipitate 


272  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

dissolved  in  hydrochloric  acid  with  the  addition  of  potassium 
chlorate.  This  solution  is  treated  with  ammonia  in  excess, 
and  the  arsenic  acid  precipitated  as  magnesium  ammonium 
arsenate  with  magnesium  mixture. 

The  precipitate  may  be  dried,  at  110°,  on  a  weighed  filter 
and  weighed,  or  converted  into  magnesium  pyroarsenate  by 
careful  ignition  in  a  porcelain  crucible. 

EXPERIMENT. 

Used  about  1  g  of  antimony  potassium  tartrate,  1  g 
sodium  arsenate,  80  cc  sodium  sulphide  solution,  and  2.5  g 
sodium  hydroxide. 


Current- 
density, 
Amperes. 

Potential- 
difference, 
Volts. 

Temp. 

Time, 
hrs.  m. 

Found 
Sb, 
g. 

Taken 
Sb, 
g. 

1.55-1 

.5 

1 

.75-1 

.1 

54-57° 

3 

30 

0 

.3778 

0, 

,3773 

1.60-1 

.5 

2, 

,10-1 

.45 

25-38° 

6 

— 

0 

.3770 

0. 

3773 

0.5  -0 

.4 

1 

.75-0 

.8 

21-24° 

overnight 

0 

.3770 

0 

,3770 

Antimony — Tin — Arsenic. 
LITERATURE  I 

Classen,  Ber.  deutsch.  chem.  Ges.,  17,  2245  (1884); 

ibid.,  18,  1110  (1885);  ibid.,  28,  2060  (1895). 
Classen  and  Ludwig,  ibid.,  19,  323  (1886). 

If  arsenic  is  present  as  arsenic  acid,  antimony  alone  is 
precipitated  from  a  concentrated  alkaline  solution  of  the  three 
metals  in  sodium  sulphide;  tin  and  arsenic  remain  in  solution. 
The  arsenic  is  converted  into  arsenic  acid,  and  the  antimony 
precipitated,  exactly  as  heretofore  described. 

For  the  separation  of  tin  from  arsenic,  the  solution  poured 
off  from  the  antimony  is  treated  with  dilute  sulphuric  or 
hydrochloric  acid  to  decompose  the  sulpho-salts,  the  mixture 
of  arsenic  sulphide,  tin  sulphide,  and  sulphur  is  filtered  off, 
oxidised  with  hydrochloric  acid  and  potassium  chlorate,  and 
the  arsenic  separated  as  described  on  p.  273.  To  determine 


ANTIMONY.  273 

the  tin,  the  solution  freed  from  arsenic  is  saturated  with  hy- 
drogen sulphide,  filtered,  and  the  tin  sulphide  dissolved  in 
ammonium  sulphide.  The  tin  is  determined  electrolytically 
as  directed,  p.  215. 

In  the  analysis  of  a  substance  which  contains  arsenic, 
antimony,  and  tin,  the  arsenic  may  also  be  first  eliminated 
according  to  the  method  of  E.  Fischer-Hufschmidt,  simpli- 
fied by  R.  Ludwig  and  the  author,  *  and  antimony  and  tin 
separated  in  the  arsenic-free  solution. 

If  the  sulphides  of  the  metals  are  to  be  separated,  they 
are  oxidised  with  concentrated  hydrochloric  acid  and  potas- 
sium chlorate  and  the  acid  evaporated  on  the  water-bath. 
The  residue  is  washed  with  fuming  hydrochloric  acid  into 
a  flask  of  500-600  cc  capacity,!  treated  with  20-25  cc  of  a 
saturated  solution  of  ferrous  chloride,  or,  better,  with  about 
25  g  of  ammonium  ferrous  sulphate  [FeSO^NH^SC^.GILjO], 
and  fuming  hydrochloric  acid  added  till  the  volume  is  150 
to  200  cc.  A  strong  current  of  hydrochloric  acid  gas  is  now 
passed  into  the  solution  and  kept  up  for  at  least  half  an  hour 
after  the  solution  seems  fully  saturated.  Then  the  solution 
is  reduced  to  about  50  cc  by  distilling  off  the  liquid,  without 
a  condenser,  in  a  stream  of  hydrogen  chloride  gas.  A  flask 
of  about  1  liter  capacity,  containing  400-500  cc  water,  is 
used  as  a  receiver.  If  the  flask  is  well  cooled  during  the 
distillation,  not  a  trace  of  arsenic  passes  over  into  a  second 
receiver,  even  when  as  much  as  0.5  g,  reckoned  as  As203,  is 
present. 

The  arsenic  in  the  distillate  may  either  be  saturated  with 
sodium  carbonate  and  titrated  with  iodine  solution  or  pre- 
cipitated as  As2S3  with  hydrogen  sulphide,  and  determined 

*  Ber.  d.  ch.  Ges.,  18,  1110  (1885). 

f  A  convenient  apparatus  is  illustrated  in  the  author's  "Handbuch  der 
Quantitative  Analyse,"  4th  edition,  p.  78. 


274  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

as  such  on  a  weighed  filter,  or  the  arsenic  can  be  calculated 
by  determining  the  amount  of  sulphur  in  the  precipitate. 
The  process  in  the  latter  case  is  as  follows:  The  distillate 
is  mixed  with  twice  its  volume  of  water,  air  expelled  by 
a  strong  current  of  carbon  dioxide,  and  the  arsenic  pre- 
cipitated by  passing  in  pure  hydrogen  sulphide  gas.  The 
excess  of  hydrogen  sulphide  is  removed  by  passing  a  strong 
current  of  carbon  dioxide  till  lead  acetate  paper  is  not  colored 
by  the  escaping  gases.  The  arsenic  sulphide  is  allowed  to 
subside,  and  the  clear  solution  siphoned  off.  The  remaining 
strongly  acid  solution  is  saturated  with  ammonia,  which  dis- 
solves the  arsenic  sulphide;  the  solution  is  then  boiled  with 
an  excess  of  hydrogen  peroxide  free  from  sulphuric  acid.  The 
solution  is  acidified  with  hydrochloric  acid,  and  the  sul- 
phuric acid  produced  by  the  action  of  the  hydrogen  peroxide 
determined  as  barium  sulphate  in  the  usual  way  (Classen). 

To  determine  the  antimony  and  tin,  the  strong  acid  solu- 
tion in  the  flask,  which  contains  the  iron,  is  diluted  with 
three  times  its  volume  of  water.  Antimony  and  tin  are  pre- 
cipitated with  hydrogen  sulphide.  After  the  precipitate  has 
subsided,  the  clear  solution  is  poured  on  a  filter,  the  pre- 
cipitate washed  several  times  by  decantation,  and  afterwards 
on  the  filter,  with  hot  water,  till  free  from  hydrochloric  acid. 
Portions  of  the  sulphides  often  adhere  to  the  walls  of  the  flask 
in  which  the  precipitation  took  place.  These  are  washed  out 
with  concentrated  sodium  sulphide  solution  and  the  solution 
is  poured  on  the  filter  containing  the  sulphides.  The  filtrate 
is  collected  in  a  weighed  platinum  dish.  The  filter,  on  which 
some  iron  sulphide  always  remains  after  the  solution  of  the 
antimony  and  tin  sulphides,  is  washed  with  sodium  sulphide 
solution,  the  necessary  amount  of  sodium  hydroxide  is  added 
to  the  filtrate,  and  the  antimony  and  tin  are  separated  electro- 
lytically  as  already  directed. 


SEPARATION    OF   GOLD    FROM    OTHER    METALS.          275 
TIN— PHOSPHORIC  ACID. 

In  the  determination  of  metals  in  the  presence  of  phos- 
phoric acid  the  latter  is  often  removed  as  tin  phosphate. 
The  phosphoric  acid  is  then  usually  determined  in  a  separate 
portion,  as  its  determination  in  the  tin  precipitate  is  too 
difficult  and  slow  a  process.  The  precipitate  of  tin  oxide 
and  tin  phosphate  may,  however,  be  dissolved  by  digesting 
with  ammonium  sulphide,  the  solution  diluted,  the  tin  pre- 
cipitated by  electrolysis,  and  the  phosphoric  acid  determined 
as  usual. 

SEPARATION  OF  GOLD  FROM  OTHER  METALS. 
LITERATURE  I 

Smith  and  Muhr,  Ber.  deutsch.  chem.  Ges.,  23,  2175  (1890). 

Smith,  Am.  Chem.  Journ.,  13,  206  (1892). 

Smith  and  Wallace,  Ber.  deutsch.  ch^em.  Ges.,  25,  779  (1892); 

Journ.  Anal.  Chem.,  6,  87  (1892). 
Smith  and  Muhr,  Am.  Chem.  Journ.,  13,  417  (1892). 
Kollock,  Journ.  Am.  Chem.  Soc.,  21,  911  (1899). 

Edgar  F.  Smith  has  made  an  exhaustive  study  of  the 
action  of  the  electric  current  on  solutions  containing  the 
cyanides  of  the  metals,  and  has  applied  this  method  to  the 
separation  of  gold  from  palladium,  copper,  nickel,  zinc,  and 
platinum. 

Kollock,  who  carried  out  a  series  of  experiments  in 
Smith's  laboratory,  has  published  the  following  details  for 
this  method: 

Gold- Palladium. — 2  grams  of  potassium  cyanide  were 
added  to  a  solution  containing  gold  chloride  (  =  0.1256  g  Au) 
and  palladium  chloride  (  =  0.1  g  Pd).  The  solution  was 
diluted  to  125-250  cc,  warmed  to  65°,  and  electrolysed 


276  QUANTITATIVE    ANALYSIS    BY   ELECTROLYSIS. 

with  a  current  of  ND100  =  0.03-0.06  ampere  and  a  potential- 
difference  of  2.5  volts.  The  gold  was  precipitated  free  from 
palladium  in  6  hours. 

Gold-Copper. — 2  grams  of  potassium  cyanide  were  added 
to  a  solution  containing  gold  chloride  (  =  0.1665  g  Au)  and 
copper  sulphate  (  =  0.1  g  Cu).  The  solution  was  diluted 
to  250  cc  and  electrolysed  at  65°  with  a  current  of  ND100  = 
0.05-0.07  ampere  and  a  difference  of  potential  between  the 
electrodes  of  1.7-1.9  volts.  The  gold  was  completely  pre- 
cipitated in  about  three  hours. 

Gold- Nickel. — 4  grams  of  potassium  cyanide  were  added 
to  a  solution  containing  0.1610  g  gold  as  chloride  and  nickel 
nitrate  (  =  0.1  g  Ni).  The  solution  was  diluted  to  125  cc, 
maintained  at  a  temperature  of  65°,  and  electrolysed  with 
a  current  of  ND100  =  0.05  ampere  and  a  potential-difference 
of  1.6  volts.  After  7  hours  the  gold  was  entirely  precipitated. 

Gold-Cobalt. — The  conditions  of  the  experiment  were 
similar  to  those  for  the  separation  of  gold  and  nickel. 

Gold-Zinc. — The  solution  contained  0.1608  g  of  gold  as 
chloride  and  zinc  sulphate  (  =  0.1  g  Zn).  To  this  4  g 
potassium  cyanide  were  added,  it  was  diluted  to  125-250 
cc,  and  electrolysed  at  60°  with  a  current  of  ND100  =  0.06 
ampere  and  a  potential-difference  of  2.7  volts.  The  com- 
plete precipitation  of  the  gold  required  7  hours. 

Gold-Platinum. — The  solution  contained  gold  and  plat- 
inum chlorides  (Au  =  0.1576  g)  (Pt  =  0.1  g).  To  this  so- 
lution 1.5  g  of  potassium  cyanide  was  added,  the  volume 
was  increased  to  250  cc  by  diluting  with  water,  and  the 
electrolysis  was  conducted  at  70°  with  a  current  of  ND100  = 
0.01  ampere  and  a  potential-difference  of  2.7  volts.  In  three 
hours  the  gold  was  completely  precipitated. 


POTASSIUM — SODIUM.       SODIUM — AMMONIUM.  277 

PLATINUM— IRIDIUM. 

As  stated  on  page  218,  platinum  can  be  separated  from  a 
hydrochloric  acid  solution  by  a  current  of  ND100  =  0.05  am- 
pere and  a  potential-difference  of  1.2  volts. 

This  property  of  platinum  may  be  used  for  separating  it 
from  indium,  which  under  similar  conditions  remains  in  solu- 
tion. 

The  platinum  is  deposited  free  from  indium  (Classen). 

POTASSIUM— SODIUM. 

The  ordinary  method  of  determining  potassium  and  so- 
dium in  the  same  solution  is  to  weigh  the  mixed  chlorides, 
and  the  potassium  as  potassium  platinic  chloride ;  the  sodium 
is  thus  determined  by  difference.  The  errors  of  the  work, 
therefore,  all  fall  on  the  sodium.  The  potassium  may  be  de- 
termined, as  already  directed  (p.  218),  by  precipitating  as 
potassium  platinic  chloride  and  determining  the  platinum 
in  the  latter  by  electrolysis.  To  determine  the  sodium 
directly,  the  nitrate  from  the  potassium  platinic  chloride  is 
evaporated  on  the  water-bath  to  remove  alcohol,  the  residue 
dissolved  in  water  with  the  addition  of  a  little  hydrochloric 
acid,  and  the  platinum  deposited  by  electrolysis.  The  sodium 
chloride  in  the  solution  poured  off  from  the  platinum  is  de- 
termined by  evaporating  to  dryness  and  weighing  the  residue. 

SODIUM— AMMONIUM. 

The  direct  determination  of  both  is  accomplished  as  with 
potassium  and  sodium;  the  ammonium  is  precipitated  as 
ammonium  platinic  chloride  and  the  process  conducted  as 
described  above. 


278  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS. 


SECTION  V. 

SEPARATION  OF  THE  HALOGENS. 

LITERATURE : 
Specketer,  Zeit.  f.  Elektrochem.,  4,  539  (1898). 

The  method  of  separating  chlorine,  bromine,  and  iodine 
suggested  by  Specketer  depends  upon  the  fact  that  the 
potential-difference  required  to  effect  the  separation  of  iodine, 
in  the  form  of  silver  iodide,  on  a  silver  anode  is  lower  than  that 
required  to  separate  bromine  as  silver  bromide,  which  in  turn 
is  lower  than  that  required  to  separate  chlorine  as  silver 
chloride. 

In  Specketer 's  experiments  the  salts  (KI,  KBr,  KC1) 
were  dissolved  in  a  normal  sulphuric  acid  solution,  and  this 
mixture  was  electrolysed.  For  the  electrolytic  cell  a  narrow 
cylindrical  glass  vessel  closed  with  a  cork  stopper  was  used. 
A  glass  tube  dipping  into  the  electrolyte,  through  which  a 
constant  stream  ^of  hydrogen  was  passed  during  the  elec- 
trolysis, extended  through  the  stopper  to  the  bottom  of  the 
cell.  The  stopper  also  carried  the  cathode,  a  strip  of  plat- 
inum foil,  and  the  anode,  a  strip  of  gauze  made  from  pure 
silver  wire. 

The  source  of  current  was  a  Giilcher  thermopile,  the 
current  from  this  being  passed  through  a  brass  wire.  The 
electrolytic  cell  was  connected  in  shunt  between  one  terminal 
of  the  wire  and  a  sliding  contact  which  could  be  moved  along 
it  (see  p.  104).  By  this  arrangement  any  difference  of  po- 
tential less  than  3  volts  (that  of  the  thermopile)  could  be 
maintained  between  the  electrodes  in  the  electrolytic  cell. 


SEPARATION    OF    THE    HALOGENS.  279 

For  the  separation  of  iodine  from  bromine  and  chlorine  a 
potential-difference  of  0.13  volts  was  used.  Under  these 
conditions  only  iodine  was  deposited  (to  form  Agl).  For 
separating  bromine  from  chlorine  the  potential-difference 
used  was  0.35  volt,  the  bromine  being  deposited  under  these 
conditions.  Owing  to  the  speed  and  convenience  of  the 
volumetric  method,  the  chlorine  in  the  residual  solution 
was  determined  by  titration  according  to  Volhard's  method. 

It  was  found  that  the  purity  of  the  silver  used  for  the 
anode  was  very  important,  and  that  the  presence  of  even 
slight  traces  of  copper  was  detrimental  to  accurate  results, 
since  by  the  dissolving  of  copper  from  the  anode  the  results 
obtained  were  too  low. 

The  end  of  the  separation  of  a  given  halogen  was  deter- 
mined by  observing  the  fall  in  the  current-strength  as  indi- 
cated by  a  sensitive  galvanometer  connected  in  series  with 
the  cell.  When  the  separation^  was  complete,  practically 
no  current  was  observed  to  flow  through  the  electrolytic  cell 
at  the  given  difference  of  potential. 

The  mean  average  of  the  results  obtained  in  13  different 
determinations  of  iodine  was:  taken  0.1825  g,  found  0.1814 
g;  of  8  determinations  of  bromine:  taken  0.1936  g,  found 
0.1926  g. 


PART  THIRD. 
SECTION  I. 

SOME   APPLIED    EXAMPLES   OF    ELECTROCHEMICAL 

ANALYSIS.* 


BRASS. 
Alloy  of  Copper  and  Zinc  (Lead,  Tin,  Iron). 

FOR  the  complete  analysis  of  4his  alloy  a  sample  weighing 
about  0.5  g  should  be  taken.  This  is  dissolved  in  a  small 
quantity  of  dilute  nitric  acid,  and  the  solution  evaporated 
to  dryness  on  the  water-bath.  The  residue  is  moistened 
with  dilute  nitric  acid,  dissolved  in  a  small  quantity  of  hot 
water,  and  any  stannic  oxide  appearing  in  the  solution  re- 
moved by  filtration.  The  tin  can  be  determined  by  the 
ordinary  gravimetric  method  (igniting  and  weighing  the 
stannic  oxide)  or  by  electrolysis  according  to  the  method  de- 
scribed on  p.  213.  The  filtrate  from  the  oxide  of  tin  is  evap- 
orated on  the  water-bath  in  the  presence  of  a  slight  excess  of 

*  Most  of  the  applied  examples  of  electrochemical  analysis  here  given 
appeared  in  the  third  German  and  second  English  editions  of  this  work, 
but  are  not  contained  in  the  fourth  German  edition.  Owing  to  the  prac- 
tical value  in  these  examples  the  translator  has  thought  it  desirable  to 
include  them  in  the  present  edition,  and  has,  at  the  same  time,  made  such 
alterations  as  the  recent  advances  along  the  various  lines  would  seem  to 
justify. 

281 


282  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

sulphuric  acid  until  the  odor  of  nitric  acid  can  no  longer  be 
detected.  The  residue  is  moistened  with  a  few  drops  of 
dilute  sulphuric  acid,  treated  with  hot  water,  and  any 
lead  sulphate  which  appears  is  filtered  off  and  washed  with 
water  containing  a  very  small  quantity  of  sulphuric  acid. 

The  quantity  of  the  lead  sulphate  can  be  determined  by 
the  ordinary  gravimetric  method,  or  can  be  dissolved  and  the 
lead  determined  by  electrolysis  under  the  conditions  de- 
scribed on  p.  196. 

About  5  cc  of  concentrated  nitric  acid  are  now  added  to 
the  filtrate  from  the  lead  sulphate,  which  is  diluted  to  about 
150  cc,  and  the  copper  is  precipitated  by  the  method  given 
on  p.  180.  When  the  separation  of  the  copper  is  complete, 
the  solution  containing  the  zinc  is  poured  off,  and  is  again 
evaporated  on  the  water-bath  to  remove  nitric  acid.  The 
residue  is  dissolved  in  a  small  quantity  of  water  and  a  slight 
excess  of  ammonia  added,  when  any  iron  present  will  be 
precipitated  as  ferric  hydroxide  and  can  be  removed  by  fil- 
tration. Ammonium  oxalate  is  now  added  to  the  solution 
containing  the  zinc,  and  this  element  is  precipitated  as  metal 
by  the  method  described  on  p.  165.  The  same  electrode 
upon  which  the  copper  has  been  precipitated  can  be  used  for 
receiving  the  zinc.  By  this  the  necessity  of  preparing  a 
special  copper-plated  electrode  is  avoided. 

NICKEL   COIN. 
Alloy  of  Copper  and  Nickel. 

About  0.5  g  of  this  alloy  is  dissolved  in  dilute  nitric  acid, 
8  cc  of  dilute  sulphuric  acid  (50%)  added,  and  the  solution 
evaporated  on  the  water-bath  until  all  nitric  acid  is  ex- 
pelled. The  residue  is  then  dissolved  by  warming  with 
about  100  cc  of  water — which  requires  some  time  since  the 


GERMAN    SILVER.  283 

sulphate  of  nickel  dissolves  very  slowly — 5  cc  of  dilute  nitric 
acid  is  added,  the  liquid  is  diluted  to  150  cc,  and  electrolysed 
with  a  current  of  ND100  =  0.5  ampere  and  a  difference  of  poten- 
tial of  2.2  volts.  By  this  treatment  the  copper  is  completely 
precipitated  in  about  6  hours. 

After  the  removal  of  the  copper  the  solution  is  evaporated 
on  the  water-bath  to  remove  nitric  acid,  the  residue  is  dis- 
solved by  warming  with  about  80  cc  of  water,  the  solution 
is  neutralised  with  ammonia,  and  about  40  cc  of  ammonia 
(sp.  gr.  0.96)  are  added.  After  diluting  to  about  150  cc  the 
nickel  in  the  solution  is  precipitated  by  a  current  of  ND100  = 
0.5-1.5  ampere  and  a  potential-difference  of  2.8-3.3  volts. 

GERMAN  SILVER. 
Alloy  of  Copper,  Zinc,  Nickel  (Tin,  Lead). 

The  copper,  tin,  and  lead  present  in  this  alloy  can  be  de- 
termined by  the  method  given  for  the  determination  of  these 
elements  in  brass  (p.  281).  About  0.3  g  of  the  alloy  should 
be  taken. 

For  separating  the  zinc  and  nickel  the  method  described 
by  Vortmann  can  be  used.  For  this  purpose  the  solution, 
after  the  separation  of  the  copper,  is  evaporated  to  remove 
the  nitric  acid,  and,  after  dissolving  the  residue  in  water, 
the  solution  is  neutralised  with  sodium  hydroxide.  5  g  po- 
tassium sodium  tartrate  is  now  added  to  the  solution,  which 
is  made  strongly  alkaline  with  sodium  hydroxide,  and  the 
zinc  is  precipitated  with  a  current  of  ND100  =  0.3-0.6  ampere 
and  a  potential  difference  of  2  volts.  The  zinc  can  be  pre- 
cipitated on  the  electrode  bearing  the  copper  precipitate. 
In  this  operation  oxide  of  nickel  may  separate  on  the  anode, 
or  may  appear  in  the  solution  in  sufficient  quantities  to 
slightly  discolor  the  precipitated  zinc.  This  may  be  pre- 


284  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

vented  by  adding  a  small  quantity  of  potassium  iodide  to 
the  solution. 

According  to  Neumann*  the  nickel  remaining  in  the 
solution  after  the  separation  of  the  zinc,  can  be  determined 
by  slightly  acidifying  the  solution  with  sulphuric  acid,  adding 
the  proper  excess  of  ammonia  and  precipitating  the  nickel  by 
the  method  described  on  page  158. 

Since  the  separation  of  nickel  and  zinc  by  electrolysis  is 
not  in  all  cases  entirely  satisfactory,  it  is  perhaps  better  to 
conduct  this  separation  in  a  formic-acid  solution  by  treat- 
ment with  hydrogen  sulphide. 

This  method  of  separation  was  first  proposed  by  Hampe,f 
and  the  following  modification  by  Prof.  H.  L.  Wells  of  the 
Sheffield  Scientific  School  can  be  especially  recommended  in 
the  case  under  consideration. 

A  sample  of  the  alloy  weighing  1  g  is  taken  for  the  an- 
alysis, and  after  separating  the  copper  as  already  described, 
the  solution  is  evaporated  to  remove  nitric  acid,  and  the 
residue  of  sulphates  is  dissolved  in  water.  About  2  or  3  cc 
of  formic  acid  (sp.  gr.  1.12)  is  now  added  to  the  solution,  then 
enough  ammonia  to  change  the  color  to  blue,  and  finally 
25  cc  of  formic  acid  (sp.  gr.  1.12).  The  solution  is  now  diluted 
to  a  volume  of  100  cc,  heated  to  boiling,  the  source  of  heat 
removed,  and  a  rather  rapid  stream  of  hydrogen  sulphide 
passed  into  the  solution  for  about  15  minutes. 

The  precipitated  zinc  sulphide,  which  should  be  pure 
white,  is  filtered  from  the  hot  solution  and  washed  with  hot 
water.  The  filtrate  is  evaporated  to  dryness,  25  cc  each  of 
concentrated  hydrochloric  and  nitric  acid  added,  and  heated 
on  the  water-bath  until  effervescence  ceases.  The  solution 
is  then  evaporated  and  heated  until  all  traces  of  nitric  and 

*  Analytischen  Elektrolyse.     Halle,  1897. 
f  Zeit.  f.  anal.  Chem.,  24,  588  (1885). 


COPPER — ALUMINIUM    ALLOYS.  285 

hydrochloric  acid  have  been  expelled.  The  residue  is  dis- 
solved in  a  small  quantity  of  water,  the  solution  made  alka- 
line with  ammonia,  and  any  ferric  hydroxide  which  appears 
removed  by  filtration.  The  ferric  hydroxide  should  be 
washed,  dissolved  in  a  small  quantity  of  warm  dilute  sul- 
phuric acid,  reprecipitated  with  ammonia,  and  this  filtrate 
with  the  washings  added  to  the  filtrate  from  the  first  pre- 
cipitation. 

For  the  determination  of  the  nickel,  about  40  cc  ammonia 
(sp.  gr.  0.96)  are  added  to  the  solution  and  the  nickel  is  pre- 
cipitated under  the  conditions  given  on  p.  158. 

The  precipitate  of  zinc  sulphide  obtained  from  the  treat- 
ment with  hydrogen  sulphide  can  be  dissolved  in  a  small 
quantity  of  sulphuric  acid,  and  the  zinc  precipitated  by  elec- 
trolysis by  one  of  the  methods  given  under  Zinc  (p.  163). 


COPPER— ALUMINIUM  ALLOYS. 

For  the  special  determination  of  copper,  in  copper-alumin- 
ium alloys,  Regelsberger  *  suggests  dissolving  3-5  g  of  the 
alloy  in  nitric  acid  and  evaporating  the  solution  down  to  the 
consistency  of  sirup.  The  sample  is  diluted,  and  a  measured 
quantity  (corresponding  to  0.6-1  g  substance)  is  poured  into 
the  electrolytic  cell.  An  excellent  precipitate  is  obtained  if 
the  acid  solution  is  neutralised  with  ammonia  and  10  cc  of 
dilute  nitric  acid  (sp.  gr.  1.2)  are  added  to  200  cc  of  the 
liquid.  The  clear  solution  is  electrolysed  with  a  current- 
density  of  ND100  =  0.4  amp.  When  the  solution  is  warmed 
the  separation  is  completed  in  about  three  hours. 

*  Zeit.  f.  angew.  Chem.,  p.  473  (1891). 


286  QUANTITATIVE   ANALYSIS    BY   ELECTROLYSIS. 

BRONZE. 
Alloy  of  Copper  and  Tin. 

The  alloy  in  a  finely  divided  form  is  treated  with  aqua 
regia,  and  the  solution  is  evaporated  to  dryness.  The  residue 
is  digested  with  a  concentrated  solution  of  sodium  sulphide, 
which  dissolves  the  tin,  and  the  copper  sulphide  which  remains 
after  filtering  is  washed  thoroughly  with  sodium  sulphide  and 
then  with  hydrogen  sulphide  solution,  dissolved  in  the  proper 
quantity  of  nitric  acid,  and  the  copper  precipitated  under  the 
conditions  given  under  Copper  (p.  179). 

The  solution  of  tin  in  sodium  sulphide  is  brought  to  a 
volume  of  about  150  cc,  25-30  g  ammonium  sulphate  is 
added,  and  the  solution  is  boiled  for  about  one-half  hour  to 
convert  the  sodium  sulphide  into  ammonium  sulphide  (see 
p.  216).  The  solution  thus  obtained  is  treated  as  described 
on  p.  215. 

Accurate  results  may  also  be  obtained  *  by  treating 
0.2-0.4  g  of  the  alloy,  best  in  the  form  of  fine  turnings, 
with  6  cc  nitric  acid  (sp.  gr.  =1.5),  and  adding  3  cc  water. 
When  the  reaction  is  over,  the  solution  is  heated  to  boiling, 
diluted  with  15  cc  boiling  water,  and  the  stannic  oxide  filtered 
off.  To  the  solution  containing  the  copper,  5-10  cc  of  nitric 
acid  is  added,  and  the  copper  is  precipitated  as  directed  on 
p.  180.  The  stannic  oxide  is  dissolved  in  ammonium  sul- 
phide and  determined  electrolytically  (p.  215). 

PHOSPHOR-BRONZE. 
Alloy  of  Copper,  Tin,  Zinc,  and  Phosphorus. 

When  the  alloy  is  digested  with  concentrated  nitric  acid, 
as  stated  under  Bronze,  a  precipitate  remains,  which  consists 

*  Neumann,  1.  c. 


MANGANESE    PHOSPHOR-BRONZE. — SOLDER.  287 

of  a  mixture  of  tin  oxide  and  tin  phosphate,  with  small  quan- 
tities of  copper  oxide.  It  is  filtered  off,  washed  with  water 
containing  nitric  acid,  and  heated  with  a  concentrated  solution 
of  sodium  sulphide.  The  residue  of  copper  sulphide  is  dis- 
solved in  nitric  acid,  and  added  to  the  principal  solution. 

The  tin  is  determined  by  converting  the  sodium  sulphide 
into  ammonium  sulphide,  and  electrolysing  as  directed,  p.  215. 
The  phosphoric  acid  is  determined  in  the  filtrate  in  the  usual 
manner. 

The  nitric-acid  solution  contains  the  copper  and  zinc. 
They  are  separated  according  to  directions  for  the  analysis  of 
brass  (p.  281). 


MANGANESE   PHOSPHOR-BRONZE. 
Alloy  of  Copper,  Tin,  Zinc,  Manganese,  and  Phosphorus. 
The  process  is  similar  to  that  given  for  Phosphor-bronze ; 
the  manganese  remains  with  the  zinc,  and  is  finally  separated 
as  directed  p.  248. 

SOLDER. 

Alloy  of  Tin  and  Lead. 

About  0.4  g  of  the  alloy  in  the  form  of  small  pieces  is 
treated  with  6  cc  nitric  acid  (sp.  gr.  =  1.5)  and  3  cc  water. 
When  the  reaction  is  completed  the  solution  is  heated  to  boil- 
ing, and  diluted  with  15  cc  hot  water,  the  precipitate  of  stan- 
nic oxide  allowed  to  settle,  filtered  off,  and  washed  with  water 
containing  a  little  nitric  acid.  The  stannic  oxide  may  be 
determined  gravimetrically,  or  may  be  dissolved  in  ammonium 
sulphide  and  determined  by  electrolysis  according  to  the 
directions  given  on  page  215.  The  lead  contained  in  the 
filtrate  may  be  determined  by  the  method  given  on  page  195. 


288  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

WOOD'S  METAL. 
Alloy  of  Tin,  Lead,  Bismuth,  and  Cadmium. 

The  alloy  is  treated  similarly  to  solder,  the  tin  being  sepa- 
rated and  determined  in  the  same  manner.  Since  it  is  impos- 
sible to  separate  lead  and  bismuth  by  electrolysis,  it  is  neces- 
sary to  evaporate  the  solution  to  a  sirup  on  the  water-bath, 
add  water  and  repeat  the  operation  until  the  odor  of  nitric 
acid  can  no  longer  be  detected.  The  solution  is  then  treated 
with  dilute  ammonium  nitrate  solution,  and  the  basic  bismuth 
nitrate  is  filtered  off.*  A  sufficient  excess  of  nitric  acid  is 
added  to  the  filtrate,  and  the  lead  is  determined  by  electroly- 
sis. The  cadmium  is  precipitated  by  one  of  the  methods 
given  under  Cadmium  (p.  188). 

HARD   LEAD.     TYPE-METAL. 
Alloy  of  Lead  and  Antimony  (Copper). 

For  the  electrolytic  determination  of  the  metals  Neu- 
mann and  Nissenson  |  recommend  that  2.5  g  of  the  alloy 
be  dissolved  by  warming  in  a  mixture  of  10  g  tartaric  acid, 
4  cc  nitric  acid  (sp.  gr.  1.4)  and  15  cc  water.  4  cc  concen- 
trated sulphuric  acid  are  then  added,  the  solution  is  diluted 
somewhat  with  water,  allowed  to  cool,  and  further  diluted 
to  exactly  250  cc  (in  a  graduated  flask).  After  standing  for 
some  time  the  lead  sulphate  will  be  completely 'precipitated 
on  the  bottom  of  the  flask,  and  the  solution  will  have  become 
quite  clear.  50  cc  of  the  clear  solution  is  now  removed  with 
a  pipette,  made  strongly  alkaline  with  sodium  hydroxide, 
50  cc  of  a  saturated  solution  of  sodium  sulphide  added, 
the  solution  heated  to  boiling  and  filtered  immediately. 

*  Neumann,  Analytischen  Elektrolyse.     Halle,  1897. 
f  Chem.  Ztg.,  No.  49  (1895). 


ANTI-FRICTION    METAL.  289 

The  precipitate  (copper  sulphide  and  traces  of  lead  sulphide) 
is  washed  with  dilute  sodium  sulphide  solution,  and  the 
nitrate  electrolysed  according  to  the  method  given  on  p.  269. 
The  copper  is  determined  by  dissolving  the  precipitate  of 
copper  sulphide  in  nitric  acid,  and  precipitating  the  copper 
by  electrolysis  (p.  180). 

The  per  cent,  of  lead  present  can  be  determined  by  differ- 
ence, but  if  its  direct  determination  is  required  0.5  g  of  the 
alloy  can  be  taken  and  the  copper  sulphate  determined  gravi- 
metrically ;  it  is,  however,  more  satisfactory  to  treat  the  first 
solution  of  the  metals  directly  with  sodium  hydroxide  and 
sodium  sulphide,  and  to  dissolve  the  precipitate,  consisting 
of  the  sulphides  of  lead  and  copper,  in  nitric  acid.  This  solu- 
tion is  then  treated  as  described  on  p.  256. 

ANTI-FRICTION  METAL. 
Alloy  of  Lead,  Antimony,  and  Tin  (Copper). 
The  analysis  of  this  material  is  conducted  in  much  the 
same  manner  as  the  analysis  of  Type-metal.  To  the  solution 
containing  the  copper,  antimony,  and  tin,  a  slight  excess  of 
sodium  hydroxide  and  sodium  sulphide  are  added,  and  copper, 
if  present,  is  precipitated  as  sulphide.  This  precipitate  is 
filtered  off  and  washed  first  with  a 'saturated  solution  of 
sodium  sulphide,  and  finally  with  water  containing  hydrogen 
sulphide.  The  washings  with  sodium  sulphide  are  added  to 
the  filtrate,  which  should  ultimately  contain  80  cc  of  saturated 
sodium  sulphide  solution  and  an  excess  of  1-2  g  of  sodium 
hydroxide.  The  antimony  and  tin  in  the  filtrate  are  then 
separated  and  determined  according  to  the  directions  on  p. 
269. 

ALLOY  OF  ANTIMONY  AND  TIN. 

The  method  of  analysis  has  been  already  given  on  p.  268. 
The  alloy  is  oxidised  with  nitric  acid,  and  the  residue,  after 


290  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

evaporation,  dissolved  in  a  concentrated  solution  of  sodium 
sulphide,  sodium  hydroxide  added,  and  the  process  followed 
throughout  as  given  on  p.  269. 

ALLOY  OF  ANTIMONY  AND  ARSENIC. 

It  has  already  been  stated  (p.  271)  that  the  two  metals 
can  be  separated  under  conditions  similar  to  those  in  the 
separation  of  antimony  from  tin;  the  method  requires  the 
arsenic  to  be  oxidised  -to  arsenic  acid.  The  alloy  is  digested 
with  aqua  regia,  the  acid  removed  by  evaporation,  the  residue 
dissolved  in  concentrated  sodium  sulphide,  sodium  hydroxide 
added,  and  the  directions  given  on  p.  271  followed  throughout. 


CINNABAR. 

Constituents:  Mercury,  Manganese,  Copper,  Alumina,  Iron, 
Calcium,  Sulphur. 

The  mineral  is  decomposed  by  heating  with  aqua  regia, 
the  solution  evaporated  on  the  water-bath,  and  the  metals 
converted  into  nitrates  by  repeated  evaporation  with  nitric 
acid.  Mercury  and  copper  are  precipitated  from  the  nitric- 
acid  solution  (p.  204),  the  two  metals  redissolved  in  nitric 
acid,  converted  into  the  double  cyanides,  and  determined 
according  to  the  directions  on  p.  260.  The  small  amount  of 
manganese  present  is  precipitated,  as  dioxide,  in  the  elec- 
trolytic process,  and  may  be  weighed  as  such. 

To  determine  iron,  aluminium,  and  calcium,  the  solution 
decanted  from  the  metals  is  evaporated  to  dryness  on  the 
water-bath,  the  nitric  acid  removed  by  repeated  evaporation 
with  hydrochloric  acid,  the  weak  acid  solution  of  the  residue 
treated  with  ammonium  oxalate  in  great  excess,  calcium 
oxalate  filtered  off,  and  iron  and  aluminium  determined  as 
directed,  p.  232. 


MOLYBDENITE.  291 

For  the  determination  of  mercury  in  this  mineral  Smith 
and  Wallace  *  treat  about  0.2  gram  of  the  finely  pulverised 
sample  with  20-25  cc  of  a  solution  of  sodium  sulphide  solu- 
tion (sp.  gr.  1.22).  The  solution  obtained  is  diluted  to  a 
volume  of  125  cc  and  electrolysed  with  a  current  of  ND100  = 
0.12  ampere  at  a  temperature  of  70°.  The  mercury  is  com- 
pletely precipitated  (see  also  p.  204). 


MOLYBDENITE. 

For  the  determination  of  molybdenum  and  sulphur  in 
this  mineral  Kollock  and  Smith  have  published  the  follow- 
ing directions :  f 

The  pulverised  sample  is  fused  with  a  mixture  of  alkali 
carbonate  and  nitrate,  which  results  in  the  formation  of 
alkali  molybdenate  and  sulphate.  For  the  determination  of 
molybdenum  the  fusion  is  dissolved  in  water,  and  the  solu- 
tion is  acidified  with  sulphuric  acid  so  that  an  excess  of  about 
0. 1-0.2  cc  (cone.)  sulphuric  acid  is  present.  The  molybdenum 
is  then  determined  by  electrolysis  as  directed  on  p.  219. 

For  the  determination  of  both  molybdenum  and  sulphur 
the  solution  obtained  by  digesting  the  fusion  with  water  is 
filtered  to  remove  any  insoluble  oxides,  acidified  with  acetic 
acid,  boiled  to  expel  carbon  dioxide,  and  electrolysed  at 
80-85°  with  a  current  of  ND100  =  0.05-0.07  ampere  and  a 
potential  of  2.5-4.4  volts.  In  from  3  to  8  hours  the  molyb- 
denum will  be  completely  precipitated  as  hydrated  oxide. 

The  sulphur,  existing  as  sulphuric  acid  in  the  solution 
poured  off  from  the  molybdenum,  is  determined  gravimetri- 
cally  by  precipitation  as  barium  sulphate. 

*  Journ.  Am.  Chem.  Soc.,  18,  169  (1895). 
f  Ibid.,  23,  669  (1901). 


292  QUANTITATIVE    ANALYSIS    BY    ELECTROLYSIS. 

DETERMINATION  OF  COPPER  IN   SULPHIDE  ORES. 

According  to  Heidenreich  *  2-5  g  of  ore  are  treated  with 
aqua  regia,  and  the  solution  is  evaporated  to  dryriess.  The 
residue  is  taken  up  with  5  cc  dilute  hydrochloric  acid  and  10 
cc  water,  transferred  to  a  flask,  diluted  to  100  cc,  and  warmed 
with  an  excess  of  sheet  aluminium.  When  the  solution  is 
colorless,  showing  that  all  iron  is  reduced,  the  residue  (Al 
and  Cu)  is  filtered  off  and  washed  until  free  from  chlorides. 
The  filter-paper  is  incinerated,  the  copper  and  aluminium 
dissolved  in  nitric  acid,  and  the  copper  separated  by  elec- 
trolysis (p.  180).  The  method  is  stated  to  be  both  accurate 

and  rapid. 

COPPER  AND  LEAD  IN  COPPER  MATTE. 

H.  Nissenson,  who  employed  the  method  described  on 
p.  258  for  determining  the  copper  and  lead  in  copper  matte, 
gives  the  following  directions  for  carrying  out  the  analysis: 

One  gram  of  copper  matte  is  dissolved  in  30  cc  nitric 
acid  (sp.  gr.  1.4)  and  the  resulting  solution  is  diluted  to  180 
cc.  The  electrolysis  is  so  conducted  that  the  lead  is  precipi- 
tated as  dioxide  on  the  platinum  dish,  a  perforated  plati- 
num disk  which  serves  as  cathode  receiving  the  copper. 
The  electrolysis  is  conducted  at  ordinary  room  temperature 
and  is  started  .with  a  current  of  ND100  =  0.5  ampere,  which 
at  the  end  of  an  hour  is  increased  to  1.5-2  amperes.  The 
copper  and  lead  are  both  completely  precipitated  in  6-7 
hours. 

For  technical  analyses,  where  the  determination  is  con- 
ducted in  nitric-acid  solutions,  the  presence  of  small  quan- 
tities of  silver  and  bismuth  can  be  neglected.  Where  the 
solution  contains  arsenic,  selenium,  or  manganese,  even  in 
very  small  quantities,  the  results  obtained  by  the  foregoing 
method  are  not  accurate. 

*  Zeit.  f.  anal.  Chem.,  40,  15  (1901). 


REAGENTS.  293 

SECTION  II. 

REAGENTS. 

POTASSIUM     OXALATE. 

The  crystallised  potassium  oxalate  of  commerce  always 
contains  determinable  quantities  of  iron  and  lead.  To  purify 
it  one  part  of  the  salt  is  dissolved  in  three  parts  of  water  in 
a  porcelain  dish,  and  ammonium  sulphide  is  added  drop  by 
drop  as  long  as  a  precipitate  forms.  The  solution  is  now 
heated  on  the  water-bath  till  the  precipitate  settles,  and 
filtered  through  a  plaited  filter.  To  decompose  the  slight 
excess  of  ammonium  sulphide  a  current  of  air  is  conducted 
through  the  solution  till  it  is  perfectly  colorless,  and  no 
longer  gives  a  reaction  with  .sodium  nitroprusside.  The 
separated  sulphur  is  allowed  to  settle,  and  the  clear  solution 
siphoned  off. 

AMMONIUM    OXALATE. 

The  same  impurities  are  present  as  in  potassium  oxalate. 
The  salt  is  purified  by  precipitating  the  hot  saturated  solution 
with  ammonium  sulphide.  It  is  heated  until  the  precipitate 
coheres  together,  and  filtered  hot  by  the  use  of  a  water- jacketed 
funnel.  The  greater  part  of  the  ammonium  oxalate  crystal- 
lises from  the  filtrate  on  cooling.  The  solution  is  poured  off, 
and  the  crystals  dried  by  placing  them  in  a  funnel  stopped 
with  asbestos,  and  connecting  with  a  filter-pump. 

OXALIC    ACID. 

The  impurities  are  similar  to  those  of  the  alkali  oxalates; 
it  is  purified  by  repeated  recrystallisation. 


294  QUANTITATIVE   ANALYSIS   BY   ELECTROLYSIS.  ' 

TARTARIC    ACID. 

This  substance  often  contains  considerable  quantities  of 
lead  and  iron  salts.  It  is  best  purified  by  dissolving  to  a 
concentrated  solution  in  water,  treating  with  hydrogen 
sulphide,  filtering  off  any  precipitate  and  removing  the  excess 
of  hydrogen  sulphide  by  blowing  air  through  the  filtrate.  The 
tartaric  acid  can  be  used  in  the  form  of  the  concentrated 
solution  or  can  be  crystallised  out  by  further  concentration. 

AMMONIUM    SULPHATE. 

The  method  of  purifying  this  salt  is  similar  to  that  de- 
scribed for  the  purification  of  ammonium  oxalate. 

SODIUM    SULPHIDE. 

The  crystallised  sodium  sulphide  of  commerce  is  not  only 
exceedingly  impure,  but  is  frequently  a  mixture  of  poly- 
sulphides  and  sodium  hydroxide.  The  presence  of  the 
latter  explains  that  of  alumina,  which  is  always  found 
in  abundance.  If  commercial  sodium  sulphide  is  used,  its 
solution  must  first  be  completely  saturated,  without  access 
of  air,  with  hydrogen  sulphide  gas.  It  is  better,  however, 
to  prepare  the  substance  directly,  in  which  case  the  process 
is  as  follows:  Sodium  hydroxide  purified  by  alcohol  is  dis- 
solved in  water  to  a  solution  of  sp.  gr.  1.35.  The  solution 
is  divided  into  two  equal  parts,  and  one-half,  with  exclusion 
of  air,  saturated  with  the  purest  possible  hydrogen  sulphide 
gas  till  the  volume  ceases  to  increase.  The  hydrogen  sul- 
phide is  purified  by  passing  it  through  a  wash-bottle  of  water, 
and  several  tubes  filled  with  cotton-wool  or  wadding.  When 
completely  saturated  the  solution  is  filtered  from  the  precipi- 


ALCOHOL.  295 

tate  formed,  and  mixed  with  the  other  half  of  the  sodium 
hydroxide  solution.  Hydrogen  sulphide  is  again  passed  into 
the  mixture,  with  exclusion  of  air,  and  it  is  filtered  again. 
The  nearly  colorless  filtrate  is  evaporated  in  a  capacious 
platinum  or  porcelain  dish,  over  a  strong  free  flame  as  quickly 
as  possible.  It  boils  without  bumping  if  a  platinum  spiral  is 
placed  in  it.  As  soon  as  a  thin  crystalline  pellicle  forms  on 
the  surface  the  boiling  is  stopped,  and  the  solution  poured 
while  hot  into  small  flasks  with  well-ground  glass  stoppers 
which  must  be  filled  full.  It  is  best  to  completely  exclude 
the  air  by  melted  paraffine.  For  the  separation  of  antimony 
and  tin,  the  solution  should  have  a  sp.  gr.  of  1.22-1.225. 

ALCOHOL. 

The  alcohol  used  for  washing  metals  must  be  free  from 
acid,  and,  as  nearly  as  possible,  absolute.  It  is  left  standing 
in  a  large  flask,  for  twelve  hours,  over  quicklime,  and  then 
distilled  off  on  a  water-  or  steam-bath.  The  distillate  must 
leave  no  residue  on  evaporation. 


INDEX  OF  AUTHORS. 


Andrews — see  Campbell. 

Arrhenius 10  ff. 

Avery  and  Dales 153,  188 

Avery — see  Nicholson. 

Balachowsky 186,  188,  241 

Bauer — see  Classen. 

Becquerel 2,  191 

Beilstein  and  Jawein 4,  163,  167,  187,  189 

Bergmann — see  Fresenius. 

Bindschedler 202 

Blake — see  Chittenden. 

Le  Blanc 5,  40,  53 

Bloxam 2 

Boisbaudran 3,  176 

Bongartz  and  Classen 212 

Bottcher 208 

Brand 153,  157,  160,  163,  170,  186,  187,  192,  202 

Bredig 56 

Bruhnatelli 206 

Bninck 186,  258 

Campbell  and  Andrews 157,  160,  162 

"        and  Champion 212 

Carlisle — see  Nicholson. 
Cauley — see  Smith. 
Champion — see  Campbell. 

Cheney  and  Richards 157,  159 

Chittenden  and  Blake 208 

Clamond 93 

Classen. . .     5,  153,  157,  160,  161,  163,  170,  174,  177,  188,  190,  192,  201, 

209,  212,  217,  225,  226,  228,  231,  233,  235,  236,  237,  243,  246,  248, 

249,  252,  254,  255,  257,  262,  268,  271,  272,  274 

297 


298  INDEX   OF   AUTHORS. 


X-AUlEj 

Classen — see  Bongartz. 

"      and  Bauer 216 

"      and  Ludwig 202,  208,  271,  272,  273 

"      and  v.  Reiss.  153,  157,  160,  163,  170,  176,  186,  187,  208,  212,  217 

Clarke 4,  187,  202 

Claubry 2 

Coehn 241 

Cohen 194 

Cozzi 2 

Croasdale 177 

Cruikshank 1 

Dales — see  Avery. 

Danneel 94 

Despretz 2 

Drossbach 261 

Drown  and  Meckenna 153 

Ducru 217,  225,  226 

Dupre 207 

Eisenberg 166,  199,  202,  251 

Eliasberg 186,  189 

Engels.   121,  170,  171,  172,  183,  212,  214,  225,  226,  228,  231,  233,  246 

Farup 58 

Fernberger  and  Smith.  157,  160,  162,  177,  183,  202,  237,  239,  243,  246, 

247,  249,  250,  253,  254 

Fisher 2 

Foote 177 

Forster  and  Seidel 177 

Foster , .    157,  160 

Francois 202,  205 

Frankel 199,  202,  206 

"      — see  Smith. 

Fresenius 176 

Fresenius  and  Bergmann 4,  157,  158,  159,  161,  199,  200 

Freudenberg. .  .   5,  53,  202,  212,  256,  258,  259,  260,  261,  264,  266,  267 
Fulweiler  and  Smith 199,  201,  258,  260 

Gibbs 2,  5,  157,  159,  163,  176,  212 

Glaser 202 

Gobbels 217 

Goll — see  Verwer. 

Gooch  and  Medway 123,  160,  162,  177,  180,  199,  201 

Gore  .  .  .208 


IXDEX    OF    AUTHORS.  299 

PAGE 

Groger 170 

Giilcher 93 

Hampe 157,  159,  176,  191,  284 

Hansen 255 

Hardin 188,  199,  202 

Haunay 4 

Head 177,  261,  262 

Heidenreich....   153,  156,  177,  188,  205,  212,  213,  247,  249,  252,  253, 

256,  258,  259,  265,  292 
Heimrod — see  Richards. 

Herpin 118,  157,  159,  176 

Herrick .' 163 

Hofer 126 

Van't  Hoff 7 

Hollard 177,  192,  209,  261 

Hoskinson 202 

and  Smith 219 

Huldschinsky — see  Rosenheim. 

Ihle — see  Reinhardt. 

-v 

Jacobi 2 

Jawein — see  Beilstein. 

Joly  and  Leidie 218 

Jordis 163,  166 

Kaeppel 170,  174,  228,  231 

Kammerer 186,  239,  244,  250,  254,  263 

Kaufmann 149 

Kern 175 

Kiliani 5,  53,  192 

"       —see  v.  Miller. 

Kinnicutt 199 

Klapproth — see  Ost. 

v.  Klobukow Ill,  122,  123,  125 

Kneer — see  Smith. 

Kohlrausch 63 

"  and  Holborn 29,  30 

Kohn  and  Woodgate 157,  159 

Kollock 153,  177,  186,  188,  190,  199,  201,  202,  204,  206,  240,  245, 

247,  248,  252,  253,  258,  260,  264,  265,  275 

"      and  Smith 175,  176,  219,  291 

Kreichgauer 192 


300  INDEX   OF   AUTHORS. 

PAGE 

Kriiger 122,  132 

Krutwig 199,  200 

Kiister  and  Steinwehr 258 

Lecrenier 208,  211 

Lehner — see  Rising. 
Leidie — see  Joly. 

Le  Roy 157,  160 

Linn 192 

Lob 149 

Lucas 261 

Luckow 3,  4,  5,  153,  157,  159,  163,  167,  170,  175,  176,  179,  186, 

187,  189,  191,  192,  193,  199,  202,  206,  208,  212,  217,  221, 

237,  258,  265 
Ludwig — see  Classen. 

Mackintosh 177 

v.  Malapert 120,  163 

Mansfeldsche  O.  Berg-  u.  Hiittendirektion 157,  159,  176 

Marie 192 

Mascazzini — see  Parodi. 

May 191,  257 

Meckenna — see  Drown. 

Medicus 192 

Medway — see  Gooch. 

Meeker 177 

Merrick 157,  159 

Miller  and  Page * 188 

v.  Miller  and  Kiliani 165 

Millot 163,  167 

Moore 153,  157,  163,  170,  177,  186,  187,  217 

Morton 2 

Moyer — see  Smith. 

Muhr — see  Smith. 

Miiller 223 

Nernst 42 

Neumann 44,  192,  197,  225,  226,  255,  284,  286,  288 

"        — see  Nissenson. 

Nicholson  and  Avery 163 

"         and  Carlisle 1 

Nickles 2 

Nissenson , 132,  192,  292 


INDEX   OF   AUTHORS.  301 

PAGE 

Nissenson  and  Neumann 192,  288 

"         and  Riist 192,  288 

Noe 93 

Oettel 58,  86,  101,  115,  157,  159,  161,  177,  182,  261,  262 

Ohl 157,  159,  176 

Ost  and  Klapproth 209,  212,  268 

Page — see  Miller. 

Parodi  and  Mascazzini 3,  153,  163,  169,  191,  208 

Paweck 163,  167 

Persoz 206 

Pfeffer ; 6 

Regelsberger 177,  285 

Reinhardt  and  Ihle 5,  163 

v.  Reiss — see  Classen. 

Revay 177,  258,  260 

Richards  and  Heimrod 20,  59 

— see  Cheney. 

Riche 119,  157,  160,  163,  170,  177,  192 

Richert .*! 3 

Riderer 163,  248 

Rimbach 188,  256 

Rising  and  Lehner 202 

Rosenheim  and  Huldschinsky 241 

Riidorff  . . .   153,  157,  160,  163,  170,  177,  183,  186,  187,  192,  199,  202, 

206,  209,  212,  217,  243,  247,  249 
Riist — see  Nissenson. 

Saltar — see  Smith. 

Sanderson 208 

Schmucker 186,  202,  261 

Schroder 179 

Schucht 157,  160,  170,  177,  186,  192,  197,  199,  218 

Schweder 157,  159,  176,  237 

Seidel — see  Forster. 

Shepard 58 

Smith 4,  100,  153,  156,  161,  163,  168,  169,  175,  177,  183,  187, 

189,  190,  191,  192,  199,  202,  205,  206,  207,  208,  217,  218, 
219,  247,  248,  251,  256,  257,  260,  266,  275 

"      and  Cauley 202 

"      and  Frankel 170,  186,  202,  258,  259 


302  INDEX    OF   AUTHORS. 

PAGE 

Smith  and  Keller 218 

"  and  Kneer  .....'. 186,  202,  251 

"  and  Moyer 177,  186,  202,  244,  256,  265 

"  and  Muhr 153,  157,  160,  187,  206,  208,  275 

"  and  Saltar 186 

"  and  Spencer 199 

"  and  Wallace..  175,  187,  188,  191,  202,  206,  242,  244,  249,  252, 

275,  291 
«  — See  Fulweiler,  Hoskinson,  Kollock,  Spare,  Thomas  and  Wallace. 

Spare  and  Smith 260,  261 

Specketer 223,  278 

Spencer — see  Smith. 

Steinwehr — see  Kiister. 

Stortenbeker 240 

Tenny 192 

Thomas  and  Smith 186 

Ullmann 177 

Ulsch 221,  222 

Verwer 153 

"       and  Goll 153 

Vortmann.,..  5,  153,  157,  160,  163,  167,  169,  186,  187,  190,  192,  202, 
208,  217,  221,  223,  226,  228,  237,  239,  241,  242,  245,  283 

Wagner 177 

Wallace — see  Smith. 

and  Smith 188,  190,  202,  253,  256 

Waller 242,  251,  268 

Warwick 163,  170,  177,  187,  192,  243 

Wells 284 

Whitefield , 223 

Wieland 186 

Wimmenauer 186 

Winkler 114,   157,  160 

v.  Wirkner 207 

Wohler 2,  218 

Woodgate — see  Kohn. 

Wrightson 3,  153,  157,  159,  163,  176,  187,  208 

Yver 4,  251 


INDEX  OF  SUBJECTS. 


PAGE 

Accumulators 79 

capacity  of 81 

charging  of 85-88 

electrolyte  for 85 

reaction  in 80 

rules  for  care  of 86 

use  and  care  of 84 

use  of  oil  on 86 

Acids,  decomposition  potential  of 41 

dissociation  of „ 14 

Alcohol,  as  reagent 295 

Alloy,  analysis  of,  containing  aluminum  and  copper 285 

antimony  and  arsenic 290 

"     lead 288 

"       "      and  tin 289 

"     tin 289 

copper   and  nickel 282 

nickel  and  zinc 283 

and  tin 286 

"        tin,  zinc  and  phosphorus 286 

(     manganese  and  phos- 
phorus    287 

"        and  zinc 281 

tin  and  lead 287 

"    lead,  bismuth  and  cadmium 288 

Aluminium,    behavior  of,  in  ammonium  oxalate  solution 174 

separation  from  cobalt 243 

iron 231 

lead 195 

nickel 246 

zinc . .                                                           .  249 


304  INDEX    OF    SUBJECTS. 


Ammonium  chloride,  electrolysis  of  solution  of 38 

determination  of 220 

oxalate,  electrolysis  of  solution  of 40 

as  reagent 293 

sulphate,  as  reagent 293 

Ampere,  definition  of  20 

Amperemanometer 56 

Amperemeter 63,  139 

for  electrochemical  analysis 66 

spring 63 

Weston 65 

Analysis,  arrangements  for 132 

Analytical  process 128 

Anions,  definition  of 13 

Anode,  definition  of 13 

Anodes 112 

rotating 123 

roughened,  use  in  determination  of  lead 194 

Anti-friction  metal,  analysis  of 289 

Antimony,  determination  of 208 

in  ammonium  sulphide  solution 209 

separation  of,  from  arsenic 271 

"      and  tin 272 

copper 179,  181,  183,  261 

lead 195 

mercury 267 

silver 266 

tin 268 

Arrhenius,  theory  of  solution 10 

Arsenic,  determination  of 217 

separation  of,  from  antimony 271 

copper 179,  181,  183,  261 

lead 196 

mercury 267 

silver 266 

Bases,  decomposition  potential  of 41 

dissociation  of 14 

Beryllium,  behavior  of,  in  ammonium  oxalate  solutions 175 

separation  from  iron 236 

Bichromate  cell 78 

Bismuth,  determination  of 186 


INDEX    OF   SUBJECTS.  305 

PAGE 

Bismuth,  separation  from  cadmium 263 

cobalt 244 

copper 183 

iron 239 

lead 195 

manganese 254 

uranium : 263 

zinc 250 

Boiling-point,  elevation  of 9 

Brass,  analysis  of 281 

Bromine,  separation  from  chlorine  and  iodine 278 

Bronze,  analysis  of 286 

Bunsen  cell 78 

Burners .' 121 

Cadmium,  determination  of 187 

in    sodium  acetate  solution 189 

solution  of  double  oxalate 188 

"        cyanide 189 

acetate 190 

containing  phosphates  . .  .  191 

sulphuric  acid  solution 191 

as  amalgam 190 

separation  from  bismuth 263 

copper 179,  183,  256 

iron 240 

lead 195,  264 

manganese 254 

mercury 264 

silver 264 

zinc x 251 

Cathions,  defined 13 

Cathode,  defined 13 

Cathodes Ill 

rotating 123 

Chlorine,  determination  of 223 

separation  from  iodine  and  bromine 278 

Chromic  acid,  electrolysis  of  solution  of 36 

Chromium,  behavior  of,  in  ammonium  oxalate  solution 175 

chloride,  electrolysis  of  solution  of 38 

separation  from  iron 233 

nickel .  246 


306  INDEX    OF   SUBJECTS. 

PAGE 

Cinnabar,  analysis  of 290 

Cobalt,  determination  of   157 

in  ammonium  oxalate  solution 158 

solution  containing   ammonium  chloride 

and  hydroxide 159 

solution  containing  ammonium  sulphate 

and  hydroxide 158 

separation  from  aluminium 243 

bismuth 244 

copper 179,  243 

chromium 243 

and  uranium 243 

gold 276 

iron 225 

lead 195,  244 

mercury 245 

nickel  241 

silver 245 

uranium 243 

zinc 242 

Concentration  cell 43 

Conductivity,  denned 24 

of  solutions 30 

effect  of  temperature  on 33 

equivalent 31 

summary  of  theory  of 32 

table 33 

unit  for 30 

table 25 

Copper,  determination  of 176 

in  ammonium  nitrate  and  hydroxide  solu- 
tion    182 

in  ammonium  oxalate  solution 177 

copper-aluminum  alloys 285 

copper  matte 292 

nitric  acid  solution 179 

sulphuric  acid  solution 181 

containing     hy- 

droxylamine  183 

containing  urea  184 

phosphate  solution 183 

sulphide  ores 292 


INDEX    OF   SUBJECTS.  307 

PAGE 

Copper,  separation  from  antimony 179,  181,  183,  261 

arsenic 179,  181,  183,  261 

bismuth 183 

cadmium  179,  183,  256 

cobalt 179,  243 

gold 276 

iron 179,  237,  239 

lead 183,  195,  256 

manganese 179,  254 

mercury 179,  183,  260 

nickel 179,  283,  246 

silver 179,  258 

tin 179 

zinc 179,  183,  249 

Copper  sulphate  solution,  electrolysis  of 37 

between  copper  electrodes      40 

voltameter 58 

Coulomb,  denned 20 

Coulombmeter 55,  58,  59 

Cover-glasses,  use  of *. 130 

Cupric  chloride  solution,  electrolysis  of 37 

Cupron  element 78 

Current 17 

Current-density,  denned 37 

importance  of,  in  electroanalytical  determinations. .     53 

Current,  source  of  . . 73 

Current-strength,  denned 16,  17 

calculation  of,  in  circuit 95-104 

containing  electrolytic  cell     48 

measurement  of 55 

method  for  regulating 95 

unit 20 

Daniell  cell 44,  75 

Decomposition  potential  of  solutions 40 

table 41 

significance  in  electrochemical  analysis 53 

Diffusion 6 

Dissociation  in  solutions 11 

laws  governing 12 

degree  of 11,  32 

Drv  cells  .  .                                                                                               ....  75 


308  INDEX    OF    SUBJECTS. 

PAGE 

Dynamos 88 

Edison-Lelande  cell 78 

Electricity,  movement  through  wires 26 

solutions 27 

unit  quantity  of 20 

Electric-lighting  currents,  use  of,  for  analysis. 108 

Electrochemical  analysis 49 

advantages  of 49 

importance  of  exact  data 54 

"  composition  of  solutions  ...  50 

equivalent  of  ions 21 

series  in  mixed  electrolytes 51 

Electrochemical  Institute  at  Aachen,  former  equipment  of 137 

present  equipment  of 141 

Electrodes 12,  110 

roughened Ill 

dish,  capacity  and  surface  of Ill 

wire-gauze 114 

preparation  of 128 

attachment  to  circuit 129 

washing  and  drying  after  use 131 

importance  of  size  of 110 

Electrolyte,  defined 12 

Electrolytes 10 

action  of  electric  current  on 34 

heated  during  electrolysis 120 

Electrolysis,  denned 12 

directions  for  conducting 130 

primary  action  during 35 

secondary  action  during 34 

vessels  for 125 

Electrolytic  solution  pressure 42 

table 44 

Electrometer,  capillary 69 

quadrant 71 

Electromotive  force  of  cell „  46 

theory  of 42 

measurement  of 70 

Faraday's  law 19 

in  electroanalytical  precipitation 52 

Freezing-point,  depression  of 9 


INDEX   OF   SUBJECTS.  309 

PAGE 

Galvanometers 59 

astatic 61 

Rowland  d' Arson val 62 

sine 60 

tangent 59 

torsion 67 

Gas  voltameter 55 

objection  to  use  of 57 

German  silver,  analysis  of 283 

Gold,  determination  of 206 

in  potassium  cyanide  solution 206-208 

sodium  sulphide  solution 208 

removal  from  cathodes 207 

separation  from  cobalt 276 

copper 276 

lead 195 

nickel 276 

palladium 275 

platinum 276 

zinc 276 

Gravity  cell 76 

Grove  cell .**. 77 

Halogens,  determination  of 223 

separation  of 278 

Hydrochloric  acid,  electrolysis  of  solution  of 38 

Internal  resistance  of  cell 46 

Ions i 11 

charges  of 13 

composition  of 13 

and  atoms,  difference  between 15 

behavior  when  neutralised  at  electrodes 16 

electrochemical  equivalent  of 21 

Iodine,  determination  of 223 

separation  from  bromine  and  chlorine 278 

Iron,  determination  of  153 

in  oxalate  solution 154,  156 

ammonium  citrate  solution 156 

separation  from  aluminium 231 

"           and  beryllium 236 

"  "     chromium . .  .235 


310  INDEX    OF    SUBJECTS. 

PAGE 

Iron,  separation  from  beryllium 236 

bismuth 239 

cadmium 240 

chromium 233 

chromium  and  uranium 235 

cobalt 225 

copper 179,  237,  238,  239 

lead 195,  239 

manganese 228-231 

nickel 226 

silver 240 

uranium 234 

zinc 228 

Irreversible  cells 79 

Kohlrausch's  law 33 

Lead,  determination  of 191 

as  amalgam 192 

metal  from  oxalate  solution 193 

peroxide  from  nitric  acid  solution 193 

separation  from  aluminium 195 

antimony 195 

arsenic 196 

cadmium 195,  264 

bismuth 195 

cobalt 195,  244 

copper 183,  195,  256 

gold 195 

iron 195,  239 

magnesium 195 

manganese 255 

mercury 195,  265 

nickel 195,  247 

silver 195,  265 

selenium 196 

zinc 252 

Lead  and  manganese  dioxides,  formation  of  50 

cell,  reaction  in  80 

hard,  analysis  of  288 

sulphate,  treatment  of,  for  analysis 196 

Leclanche  cell 73 

Lecture  room,  equipment  of 145 


INDEX    OF    SUBJECTS.  311 


Manganese,  determination  as  sulphate 230 

sulphite 231 

dioxide  from  acetic  acid  solution 171 

sodium  acetate  sol.,   172,  174 

sol.  containing  acetone.  .  174 

separation  from  bismuth 254 

cadmium 254 

copper 179,  254 

iron 228 

lead 255 

nickel 246 

zinc 248 

Manganese  phosphor-bronze,  analysis  of 287 

Membrane,  semi-permeable 7 

Meidinger  cell 76 

Mercury,  determination  of 202 

in  ammonium  oxalate  solution 203 

cinnabar 291 

insoluble  compounds 204,  205 

potassium  cyanide  solution 205,  206 

sodium  sulphide  solution 206 

solution  containing  free  acid 204 

separation  from  antimony 267 

arsenic 267 

cadmium 264 

cobalt 245 

copper 179,  183,  260 

lead 195,  265 

nickel  247 

zinc 253 

Metals,  separation  of 225 

Mho,  defined 24 

Migration  of  ions 26 

effect  on  concentration  of  solution 27 

table  of  velocities 29 

Molybdenum,  determination  of 219 

in  molybdenite 291 

Molybdenite,  analysis  of 291 

Molecular  weight  of  substances  in  solution 8 

Xickel  coin,  analysis  of 282 

Nickel,  determination  of -.  .    158 


312  INDEX    OF    SUBJECTS. 

PAGE 

Nickel,  determination  of,  in  oxalate  solution 160 

ammoniacal  solution 161,  162 

ammonium  chloride  solution 161 

potassium  cyanide  solution 162 

phosphate  solution 162 

separation  from  aluminium 246 

bismuth 244 

•  cobalt 241 

copper 179,  183,  246 

chromium 246 

gold 276 

iron 226 

lead 195,  247 

manganese 246 

mercury 247 

zinc 245 

Nitric  acid,  electrolysis  of  solution  of  38 

determination  in  nitrates   221 

Normal-density,  ND1CO,  defined 37 

Ohm,  denned 23 

Ohm's  law 22 

Osmotic  pressure 7 

of  ions  in  solution 42 

Oxalic  acid,  as  reagent 293 

electrolysis  of  solution  of 39 

Palladium,  determination  of 219 

separation  from  gold 275 

Phosphor-bronze,  analysis  of 286 

Physical  methods  of  producing  current 88 

Physical  state  of  metals,  effect  of  current-density  on 53 

Platinum,  determination  of 217 

in  sulphuric  acid  solution 218 

separation  from  gold 276 

iridium 218,  277 

Poggendorf  compensation  method    70 

Potassium,  determination  of 220 

separation  from  sodium 277 

Potassium  acetate,  electrolysis  of  solution  of 39 

cyanide,  electrolysis  of  solution  of 39 

lactate,  electrolysis  of  solution  of 39 

oxalate,  electrolysis  of  solution  of •.  .  39 


INDEX    OF    SUBJECTS.  313 


Potassium  oxalate,  as  reagent 293 

sulphate,  electrolysis  of  solution  of 34,  36 

tartrate,  electrolysis  of  solution  of 39 

valerate,  electrolysis  of  solution  of 39 

Polarisation 47 

current 47 

Polarised  electrodes 47 

Primary  elements 73 

Private  laboratory,  equipment  of 143 

Potential 16 

difference  of 18 

table  of  single 45 

measurement  of 66 

methods  for  regulating 95 

Resistance,  defined 23 

box 97 

table  of 25 

unit  of 23 

Reversible  cells 79 

Rheostats «* 97 

for  large  currents 149 

SaJ,ts,  dissociation  of 14 

decomposition  potential  of  solutions 41 

Secondary  elements 79 

Shares  of  transport  of  ions 29 

Shunt-circuit 104 

Silver,  determination  of 199 

in  cyanide  solutions 199,  201 

ammoniacal  solutions 200 

nitric  acid  solutions 200 

separation  from  antimony 266 

arsenic 266 

cadmium 264 

cobalt 245 

copper 179,  258 

iron 240 

lead 195,  265 

nickel 252 

zinc 53,  252 

Sodium,  separation  from  ammonium 277 


314  INDEX    OF    SUBJECTS. 


Sodium  succinate,  electrolysis  of  solution  of 39 

sulphate,  as  reagent 294 

Solder,  analysis  of 287 

Solutions,  conductivity  of 30 

unit  for 30 

equivalent 31 

hydrate  theory  of 6 

preparation  of,  for  analysis 129 

removal  from  electrolytic  cell ....'. 131 

theory  of 6 

Specific  resistance,  denned 24 

Stands  for  supporting  electrodes 115 

Stannous  chloride,  electrolysis  of  solution  of 40 

Storage-batteries — see  accumulators 79 

Sulphuric  acid,  electrolysis  of  solution  of 37 

Tartaric  acid,  as  reagent 294 

Thallium,  determination  of 197 

Thermopiles 92 

Giilcher 93 

regulator  for 94 

Tin,  determination  of 212 

in  ammonium  sulphide  solution 215 

oxalate  solution 212 

solution  containing  hydroxylamine 214 

separation  from  antimony 268 

copper 179 

phosphoric  acid 275 

Transformer,  direct-current 91,  146 

Type-metal,  analysis  of 288 

Uranium,  behavior  of,  in  ammonium  oxalate  solution 175 

determination  of,  as  protosesquioxide 175,  176 

separation  from  bismuth 263 

iron  ... . : 234 

nickel 246 

Van't  Hoff  s  law  for  solutions 8 

exceptions  to 10 

Vapor  pressure,  lowering  of 9 

Vessels  for  electrolysis 125 

Volt  denned 23 


INDEX    OF    SUBJECTS.  315 


Voltameter,  copper 58 

gas 55 

silver 58 

weight 58 

Voltmeter 67 

Water,  primary  dissociation  of 35 

Weight  voltameter 58 

Weston  amperemeters 65 

voltmeters 67 

Wood's  metal,  analysis  of 288 

Zinc  ammonium  oxalate,  electrolysis  of 40 

Zinc,  determination  of 163 

in  oxalate  solution 165,  166 

lactic  acid  solution 166 

potassium  cyanide  solution 167 

tartrate  solution 169 

as  amalgam 168 

acid  potassium  sulphate  solution 168 

sodium  acetate  solution 169 

citric  acid  solution 169 

separation  from  aluminium 249 

bismuth 250 

cadmium 251 

cobalt 242 

copper 179,  183,  249 

gold 276 

lead 195,  252 

manganese 248 

mercury 253 

nickel 245 

silver 53,  252 


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Woodhull's  Notes  on  Military  Hygiene i6mo, 

Young's  Simple  Elements  of  Navigation i6mo.  morocco, 


Second  Edition,  Enlarged  and  Revised i6mo,  morocco 


ASSAYING. 

Fletcher's  Practical  Instructions  in  Quantitative  Assaying  with  the  Blowpipe. 

1 2 mo,  morocco,  i  50 

Furman's  Manual  of  Practical  Assaying. 8vo,  3  oo 

Miller's  Manual  of  Assaying I2mo,  i  oo 

O'DriscolTs  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  oo 

Ricketts  and  Miller's  Notes  on  Assaying 8vo,  3  oo 

Hike's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 

Wilson's  Cyanide  Processes I2mo,  i  50 

Chlorination  Process I2mo  i  50 


ASTRONOMY. 

Comstock's  Field  Astronomy  for  Engineers 8vo,  2  50 

Craig's  Azimuth 4to,  3  50 

Doolittle's  Treatise  on  Practical  Astronomy 8vo,  4  oo 

Gore's  Elements  of  Geodesy 8vo,  2  50 

Hayford's  Text-book  of  Geodetic  Astronomy 8vo,  3  oo 

Merriman's  Elements  of  Precise  Surveying  and  Geodesy 8vo,  2  50 

*  Michie  and  Harlow's  Practical  Astronomy 8vo,  3  oo 

*  White's  Elements  of  Theoretical  and  Descriptive  Astronomy i2mo,  2  oo 


BOTANY. 

Davenport's  Statistical  Methods,  with  Special  Reference  to  Biological  Variation. 

1 6 mo,  morocco,  i  25 

Thome  and  Bennett's  Structural  and  Physiological  Botany i6mo,  2  25 

Westermaier's  Compendium  of  General  Botany.     (Schneider.) 8vo,  2  oo 


CHEMISTRY. 

Adriance's  Laboratory  Calculations  and  Specific  Gravity  Tables i2mo,  /  25 

Allen's  Tables  for  Iron  Analysis 8vo,  3  oo 

Arnold's  Compendium  of  Chemistry.     (Mandel.)     (In  preparation.') 

Austen's  Notes  for  Chemical  Students i2mo,  i  50 

Bernadou's  Smokeless  Powder. — Nitro-cellulose,  and  Theory  of  the  Cellulose 

Molecule i2mo,  2  50 

Bolton's  Quantitative  Analysis 8vo,  i  50 

*  Browning's  Introduction  to  the  Rarer  Elements 8vo,  i  50 

Brush  and  Penfield's  Manual  of  Determinative  Mineralogy 8vo,  4  oo 

Classen's  Quantitative  Chemical  Analysis  by  Electrolysis.  (Boltwood.)  . . .  .8vo  3  oo 

Cohn's  Indicators  and  Test-papers i2mo,  2  oo 

Tests  and  Reagents 8vo,  3  oo 

Copeland's  Manual  of  Bacteriology.     (In  preparation.} 

Craft's  Short  Course  in  Qualitative  Chemical  Analysis.  (Schaeffer.).  . .  .  i2mo,  2  oo 

DrechseFs  Chemical  Reactions.     (Merrill.) I2mo,  I  25 

Duhem's  Thermodynamics  and  Chemistry.     (Burgess.)     (Shortly.') 

Eissler's  Modern  High  Explosives *. 8vo,  4  oo 

3 


Eff ront's  Enzymes  and  their  Applications.     (Prescott.) 8vo,  3  oo 

Erdmann's  Introduction  to  Chemical  Preparations.     (Dunlap.) i2mo,  i  25 

Fletcher's  Practical  Instructions  in  Quantitative  Assaying  with  the  Blowpipe. 

i2mo,  morocco,  i  50 

Fowler's  Sewage  Works  Analyses i2mo,  2  oo 

Fresenius's  Manual  of  Qualitative  Chemical  Analysis.     (Wells.) 8vo,  5  oo 

Manual  of  Qualitative  Chemical  Analysis.     Parti.    Descriptive.     (Wells.) 

8vo,  3  oo 

System  of  Instruction   in    Quantitative   Chemical  Analysis.     (Cohn.) 
2  vols.    (Shortly.) 

Fuertes's  Water  and  Public  Health. i2mo,  i  50 

Furman's  Manual  of  Practical  Assaying 8vo,  3  oo 

Gill's  Gas  and  Fuel  Analysis  for  Engineers. i2mo,  i  25 

Grotenfelt's  Principles  of  Modern  Dairy  Practice.     (Wo  11.) i2mo,  2  oo 

Hammarsten's  Text-book  of  Physiological  Chemistry.     (Mandel.) 8vo,  4  oo 

Helm's  Principles  of  Mathematical  Chemistry.     (Morgan.) i2mo.  i  50 

Hinds's  Inorganic  Chemistry 8vo,  3  oo 

•  Laboratory  Manual  for  Students i2mo,  75 

Holleman's  Text-book  of  Inorganic  Chemistry.     (Cooper.) 8vo,  2  50 

Text-book  of  Organic  Chemistry.     (Walker  and  Mott.) 8vo,  2  50 

Hopkins's  Oil-chemists'  Handbook 8vo,  3  oo 

Jackson's  Directions  for  Laboratory  Work  in  Physiological  Chemistry.  .8vo,  i  oo 

Keep's  Cast  Iron 8vo,  2  50 

Ladd's  Manual  of  Quantitative  Chemical  Analysis i2mo.  i  oo 

Landauer's  Spectrum  Analysis.    (Tingle.) 8vo,  3  oo 

Lassar-Cohn's  Practical  Urinary  Analysis.     (Lorenz.) i2mo.  i  oo 

Leach's  The  Inspection  and  Analysis  of  Food  with  Special  Reference  to  State 

Control.     (In  preparation.) 

Lob's  Electrolysis  and  Electrosynthesis  of  Organic  Compounds.  (Lorenz.)  izmo,  i  oo 

Mandel's  Handbook  for  Bio-chemical  Laboratory 12 mo,  i  50 

Mason's  Water-supply.     (Considered  Principally  from  a  Sanitary  Standpoint.) 

3d  Edition,  Rewritten 8vo,  4  oo 

Examination  of  Water.     (Chemical  and  Bacteriological.) i2mo,  i  25 

Meyer's  Determination  of  Radicles  in  Carbon  Compounds.     (Tingle.).  .  1 2mo,  i  oo 

Miller's  Manual  of  Assaying i2mo,  i  oo 

Mixter's  Elementary  Text-book  of  Chemistry i2mo,  i  50 

Morgan's  Outline  of  Theory  of  Solution  and  its  Results i2mo,  i  oo 

Elements  of  Physical  Chemistry i2mo.  2  oo 

Nichols's  Water-supply.     (Considered  mainly  from  a  Chemical  and  Sanitary 

Standpoint,  1883.) 8vo,  2  50 

O'Brine's  Laboratory  Guide  in  Chemical  Analysis 8vo,  2  oo 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  oo 

Ost  and  Kolbeck's  Text-book  of  Chemical  Technology.     (Lorenz — Bozart.) 
(In  preparation.) 

*  Penfield's  Notes  on  Determinative  Mineralogy  and  Record  of  Mineral  Tests. 

8vo,  paper,  50 

Pictet's  The   Alkaloids   and   their   Chemical   Constitution.      (Biddle.)      (In 
preparation.) 

Pinner's  Introduction  to  Organic  Chemistry.     (Austen.) I2mo,  i  50 

Poole's  Calorific  Power  of  Fuels 8voy  3  oo 

*  Reisig's  Guide  to  Piece-dyeing 8vo,  25  oo 

Richards  and  Woodman's  Air,Water,  and  Food  from  a  SanitaryStandpoint .  8vo,  2  oo 

Richards's  Cost  of  Living  as  Modified  by  Sanitary  Science i2mo,  i  oo 

Cost  of  Food  a  Study  in  Dietaries i2mo,  i  oo 

•  Richards  and  Williams's  The  Dietary  Computer 8vo,  i  50 

Ricketts  and  Russell's  Skeleton  Notes  upon  Inorganic  Chemistry.     (Part  I. — 

Non-metallic  Elements.) 8vo,  morocco,  75 

Ricketts  and  Miller's  Notes  on  Assaying 8vo,  3  oo 


d  eal's  Sewage  and  the  Bacterial  Purification  of  Sewage 8vo,  3  50 

Ruddiman's  Incompatibilities  in  Prescriptions 8vo,  2  oo 

Schimpf  s  Text-book  of  Volumetric  Analysis 1 2mo ,  2  50 

Spencer's  Handbook  for  Chemists  of  Beet-sugar  Houses i6mo,  morocco,  3  oo 

Handbook  for  Sugar  Manufacturers  and  their  Chemists. .  i6mo,  morocco,  2  oo 

Stockbridge's  Rocks  and  Soils 8vo,  2  50 

*  Tillman's  Elementary  Lessons  in  Heat 8vo,  i  50 

*  Descriptive  General  Chemistry 8vo  3  oo 

Treadwell's  Qualitative  Analysis.     (HalL) 8vd,  3  oo 

Turneaure  and  Russell's  Public  Water-supplies 8vo,  5  oo 

Van  Deventer*s  Physical  Chemistry  for  Beginners.     (Boltwood.) i2mo,  i  50 

*  Walke's  Lectures  on  Explosives 8vo,  4  oo 

Wells's  Laboratory  Guide  in  Qualitative  Chemical  Analysis. 8vo,  i  50 

Short  Course  in  Inorganic  Qualitative  Chemical  Analysis  for  Engineering 

Students i2mo,  i  50 

Whipple's  Microscopy  of  Drinking-water 8vo,  3  50 

Wiechmann's  Sugar  Analysis Small  8vo.  2  50 

Wilson's  Cyanide  Processes I2mo,  i  50 

Chlorination  Process i2mo.  i  50 

Wulling's  Elementary  Course  in  Inorganic  Pharmaceutical  and  Medical  Chem- 
istry  i2mo,  2  oo 

CIVIL  ENGINEERING. 

BRIDGES  AND    ROOFS.       HYDRAULICS.      MATERIALS   OF  ENGINEERING. 
RAILWAY  ENGINEERING. 

Baker's  Engineers'  Surveying  Instruments I2mo,  3  oo 

Bixby's  Graphical  Computing  Table Paper,  19$  X  24^  inches  25 

**  Burr's  Ancient  and  Modern  Engineering  and  the  Isthmian  Canal.     (Postage 

27  cents  additional.) „ 8vo,  m  3  50 

Comstock's  Field  Astronomy  for  Engineers 8vo,  2  50 

Davis's  Elevation  and  Stadia  Tables 8vo,  i  oo 

Elliott's  Engineering  for  Land  Drainage i2mo,  i  50 

Practical  Farm  Drainage i2mo,  i  oo 

FolwelTs  Sewerage.     (Designing  and  Maintenance.) 8vo,  3  oo 

Freitag's  Architectural  Engineering.     2d  Edition,  Rewritten 8vo,  3  50 

French  and  Ives's  Stereotomy 8vo,  2  50 

Goodhue's  Municipal  Improvements i2mo,  i  75 

Goodrich's  Economic  Disposal  of  Towns'  Refuse 8vo,  3  50 

Gore's  Elements  of  Geodesy 8vo,  2  50 

Hayford's  Text-book  of  Geodetic  Astronomy 8vo,  3  oo 

Howe's  Retaining  Walls  for  Earth i2mo,  i  25 

Johnson's  Theory  and  Practice  of  Surveying Small  8vo.  4  oo 

Statics  by  Algebraic  and  Graphic  Methods 8vo,  2  oo 

Kiersted's  Sewage  Disposal i2mo,  i  25 

Laplace's  Philosophical  Essay  on  Probabilities.     (Truscott  and  Emory.)  i2mo,  2  oo 

Mahan's  Treatise  on  Civil  Engineering.     (1873.)     (Wood.) 8vo0  5  oo 

*  Descriptive  Geometry '. 8vo,  I  50 

Merriman's  Elements  of  Precise  Surveying  and  Geodesy 8vo,  2  50 

Elements  of  Sanitary  Engineering 8vo,  2  oo 

Merriman  and  Brooks's  Handbook  for  Surveyors i6mo,  morocco,    2  oo 

Nugenfs  Plane  Surveying 8vo,    3  50 

Ogden's  Sewer  Design i2mo,    2  oo 

Patton's  Treatise  on  Civil  Engineering 8vo,  half  leather,    7  50 

Reed's  Topographical  Drawing  and  Sketching 4to,    5  oo 

Rideal'sJSewage  and  the  Bacterial  Purification  of  Sewage '. 8vo,    3  50 

Siebert  and  Biggin's  Modern  Stone-cutting  and  Masonry 8vo,    i  50 

Smith's  Manual  of  Topographical  Drawing.     (McMillan.) 8vo,    2  50 

5 


Sondericker's   Graphic   Statics,   wnn   ^Applications   to   Trusses,   Beams,   ana 
Arches.     (Shortly.) 

*  Trantwine's  Civil  Engineer's  Pocket-book i6mo,  morocco,  5  oo 

Wait's  Engineering  and  Architectural  Jurisprudence 8vo,  6  oo 

Sheep,  6  50 

Law  of  Operations  Preliminary  to  Construction  in  Engineering  and  Archi- 
tecture,  8vo,  5~oo 

Sheep,  5  50 

L&W  of  Contracts 8vo,  3  oo 

Warren's  Stereotomy — Problems  in  Stone-cutting 8vo,  2  50 

Webb's  Problems  in  the  U=e  and  Adjustment  of  Engineering  Instruments. 

i6mo,  morocco,  i   25 

*  Wheeler's  Elementary  Course  of  Civil  Engineering 8vo,  4  oo 

Wilson's  Topographic  Surveying 8vo,  3  50 


BRIDGES  AND  ROOFS. 

Boiler's  Practical  Treatise  on  the  Construction  of  Iron  Highway  Bridges.  .8vo,  2  oo 

*         Thames  River  Bridge 4to,  paper,  5  oo 

Burr's  Course  on  the  Stresses  in  Bridges  and  Roof  Trusses,  Arched  Ribs,  and 

Suspension  Bridges 8vo,  3  50 

Du  Bois's  Mechanics  of  Engineering.     Vol.  II Small  4to,  10  oo 

Foster's  Treatise  on  Wooden  Trestle  Bridges 4to,  5  oo 

Fowler's  Coffer-dam  Process  for  Piers 8vo,  2  50 

Greene's  Roof  Trusses 8vo,  i  25 

Bridge  Trusses 8vo,  2  50 

Arches  in  Wood,  Iron,  and  Stone 8vo,  2  50 

Howe's  Treatise  on  Arches 8vo  4  oo 

Design  of  Simple  Roof-trusses  in  Wood  and  Steel 8vo,  2  oo 

Johnson,  Bryan,  and  Turneaure's  Theory  and  Practice  in  the  Designing  of 

Modern   Framed   Structures Small  4to,  10  oo 

Merriman  and  Jacoby's  Text-book  on  Roofs  and  Bridges: 

Part  I. — Stresses  in  Simple  Trusses 8vo,  2  50 

Part  II. — Graphic  Statics 8vo,  2  50 

Part  III. — Bridge  Design.     4th  Edition,  Rewritten 8vo,  2  50 

Part  IV. — Higher  Structures : 8vo,  2  50 

Morison's  Memphis  Bridge 4to,  10  oo 

Waddell's  De  Pontibus,  a  Pocket-book  for  Bridge  Engineers. .  .  i6mo,  morocco,  3  oo 

Specifications  for  Steel  Bridges I2mo,  i  25 

Wood's  Treatise  on  the  Theory  of  the  Construction  of  Bridges  and  Roofs.Svo,  2  oo 
Wright's  Designing  of  Draw-spans: 

Part  I.  — Plate-girder  Draws 8vo,  2  50 

Part  II. — Riveted-truss  and  Pin-connected  Long-span  Draws 8vo,  2  50 

Two  parts  in  one  volume 8vo,  3  50 


HYDRAULICS. 

Bazin's  Experiments  upon  the  Contraction  of  the  Liquid  Vein  Issuing  from  an 

Orifice.     (Trautwine.) 8vo,  2  oo 

Bovey's  Treatise  on  Hydraulics 8vo,  5  oo 

Church's  Mechanics  of  Engineering 8vo,  6  oo 

Diagrams  of  Mean  Velocity  of  Water  in  Open  Channels paper,  i  50 

Coffin's  Graphical  Solution  of  Hydraulic  Problems i6mo,  morocco,  2  50 

Flather's  Dynamometers,  and  the  Measurement  of  Power 12 mo,  3  oo 

Folwell's  Water-supply  Engineering 8vo,  4  oo 

Frizell's  Water-power 8vo,  5  oo 

6 


Fuertes's  Water  and  Public  Health i2ino,    i  50 

Water-filtration  Works i2mo,    2  50 

Ganguillet  and  Kutter's  General  Formula  for  the  Uniform  Flow  of  Water  in 

Rivers  and  Other  Channels.     (Bering  and  Trau twine.) 8vo,    4  oo 

Hazen's  Filtration  of  Public  Water-supply 8vo,    3  oo 

Hazlehurst's  Towers  and  Tanks  for  Water- works 8vo,    2  50 

Herschel's  115  Experiments  on  the  Carrying  Capacity  of  Large,  Riveted,  Metal 

Conduits 8vo,    2  oo 

Mason's    Water-supply.     (Considered    Principally  from   a    Sanitary   Stand- 
point.)    3d  Edition,  Rewritten 8vo,  4  oo 

Merriman's  Treatise  on  Hydraulics,     gth  Edition,  Rewritten 8vo,    5  oo 

*  Michie's  Elements  of  Analytical  Mechanics 8vo,    4  oo 

Schuyler's   Reservoirs   for  Irrigation,   Water-power,  and  Domestic   Water- 
supply Large  8vo,    5  oo 

**  Thomas  and  Watt's  Improvement  of  Riyers.     (Post.,  44  c.  additional),  4to,    6  oo 

Turneaure  and  Russell's  Public  Water-supplies 8vo.    5  oo 

Wegmann's  Desien  and  Construction  of  Dams 4to,    5  oo 

Water-suoolv  of  the  City  of  New  York  from  1658  to  1895 4to,  10  oo 

Weisbach's  Hydraulics  and  Hydraulic  Motors.     (Du  Bois.) 8vo,    5  oo 

Wilson's  Manual  of  Irrigation  Engineering Small  8vo,    4  oo 

Wolff's  Windmill  as  a  Prime  Mover 8vo,!  3  oo 

Wood's  Turbines 8vo,    2  50 

Elements  of  Analytical  Mechanics 8vo,    3  oo 


MATERIALS  OF  ENGINEERING. 

Baker's  Treatise  on  Masonry  Construction 8vo,  5  oo 

Roads  and  Pavements ' 8vo,  5  oo 

Black's  United  States  Public  Works Oblong  4to,  5  oo 

Bovey's  Strength  of  Materials  and  Theory  of  Structures 8vo,  7  So 

Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineering.     6th  Edi- 
tion, Rewritten 8vo,  7  5° 

Byrne's  Highway  Construction 8vo.  5  oo 

Inspection  of  the  Materials  and  Workmanship  Employed  in  Construction. 

i6mo,  3  oo 

Church's  Mechanics  of  Engineering 8vo,  6  oo 

Du  Bois's  Mechanics  of  Engineering.     VoL  I Small  4to,  7  50 

Johnson's  Materials  of  Construction Large  8vo,  6  oo 

Keep's  Cast  Iron 8vo,  2  50 

Lanza's  Applied  Mechanics 8vo,   7  50 

Martens's  Handbook  on  Testing  Materials.     (Henning.)     2  vols 8vo,  7  50 

Merrill's  Stones  for  Building  and  Decoration 8vo,  5  oo 

Merriman's  Text-book  on  the  Mechanics  of  Materials 8vo,  4  oo 

Strength  of  Materials i2mo,  i  oo 

Metcalf's  Steel     A  Manual  for  Steel-users i2mo,  2  oo 

Patton's  Practical  Treatise  on  Foundations 8vo,  5  oo 

Rockwell's  Roads  and  Pavements  in  France 12010,  i  25 

Smith's  Wire :  Its  Use  and  Manufacture Small  4to,  3  oo 

Materials  of  Machines i2tno,  i  oo 

Snow's  Principal  Species  of  Wood 8vo,  3  50 

Spalding's  Hydraulic  Cement i2mo,  2  oo 

Text-book  on  Roads  and  Pavements i2mo,  2  oo 

Thurston's  Materials  of  Engineering.     3  .Parts 8vo,  8  oo 

Part  I. — Non-metallic  Materials  of  Engineering  and  Metallurgy 8vo,  2  oo 

Part  II. — Iron  and  Steel 8vo,  3  50 

Part  III. — A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and  their 

Constituents 8vo,  2^50 

7 


Thurston's  Text-book  of  the  Materials  of  Construction 8vo,  5  oo 

Tillson's  Street  Pavements  and  Paving  Materials 8vo,  4  oo 

WaddelTs  De  Pontibus.     (A  Pocket-book  for  Bridge  Engineers.) . .  i6mo,  mor.,  3  oo 

Specifications  for  Steel  Bridges 1 2mo  ,-125 

Wood's  Treatise  on  the  Resistance  of  Materials,  and  an  Appendix  on  the  Pres- 
ervation of  Timber 8vo,  2  oo 

Elements  of  Analytical  Mechanics 8vo,  3  oo 


RAILWAY  ENGINEERING. 

Andre ws's  Handbook  for  Street  Railway  Engineers.     3X5  inches,  morocco,  i  25 

Berg's  Buildings  and  Structures  of  American  Railroads 4to,  5  oo 

Brooks's  Handbook  of  Street  Railroad  Location i6mo.  morocco,  i  50 

Butts's  Civil  Engineer's  Field-book . .  i6mo,  morocco,  2  50 

Crandall's  Transition  Curve i6mo,  morocco,  i  50 

Railway  and  Other  Earthwork  Tables 8vo,  i  50 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book.    i6mo,  morocco,  4  oo 

Dredge's  History  of  the  Pennsylvania  Railroad:   (1879) Paper,  5  oo 

*  Drinker's  Tunneling,  Explosive  Compounds,  and  Rock  Drills,  4to,  half  mor.,    25  oo 

Fisher's  Table  of  Cubic  Yards Cardboard,  25 

Godwin's  Railroad  Engineers'  Field-book  and  Explorers'  Guide i6mo,  mor.,  2  50 

Howard's  Transition  Curve  Field-book i6mo  morocco  i  50 

Hudson's  Tables  for  Calculating  the  Cubic  Contents  of  Excavations  and  Em- 
bankments    8vo,  i  oo 

Molitor  and  Beard's  Manual  for  Resident  Engineers i6mo,  i  oo 

Nagle's  Field  Manual  for  Railroad  Engineers i6mo,  morocco.  3  oo 

Philbrick's  Field  Manual  for  Engineers i6mo,  morocco,  3  oo 

Pratt  and  Alden's  Street-railway  Roadibed 8vo,  2  oo 

Searles's  Field  Engineering i6mo,  morocco,  3  oo 

Railroad  Spiral ^  . .  i6mo,  morocco  i  50 

Taylor's  Prismoidal  Formulae  and  Earthwork 8vo,  i  50 

*  Trautwine's  Method  of  Calculating  the  Cubic  Contents  of  Excavations  and 

Embankments  by  the  Aid  of  Diagrams 8vo,  2  oo 

he  Field  Practice  of  [Laying    Out    Circular    Curves    for    Railroads. 

1 2 mo,  morocco,  2  50 

*  Cross-section  Sheet Paper,  25 

Webb's  Railroad  Construction.     2 d  Edition,  Rewritten i6mo.  morocco.  5  oo 

Wellington's  Economic  Theory  of  the  Location  of  Railways Small  8vo,  5  oo 


DRAWING. 

Barr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bartlett's  Mechanical  Drawing ". 8vo,  3  oo 

Coolidge's  Manual  of  Drawing 8vo,  paper,  i  oo 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

Hill's  Text-book  on  Shades  and  Shadows,  and  Perspective 8vo,  2  oo 

Jones's  Machine  Design: 

Part  I. — Kinematics  of  Machinery 8vo,  i  50 

Part  H. — Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

Mac  Cord's  Elements  of  Descriptive  Geometry 8vo,  3  oo 

Kinematics;  or,  Practical  Mechanism 8vo,  5  oo 

Mechanical  Drawing 4to,  4  oo 

Velocity  Diagrams 8vo,  i  50 

*  Mahan's  Descriptive  Geometry  and  Stone-cutting 8vo,  i  50 

Industrial  Drawing.    (Thompson.) 8vo,  3  50 

Reed's  Topographical  Drawing  and  Sketching 4to,  5  oo 

8 


Reid's  Course  in  Mechanical  Drawing 8vo,  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design.  ,8vo.  3  oo 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Smith's  Manual  of  Topographical  Drawing.     (McMillan.) 8vo,  2  50 

Warren's  Elements  of  Plane  and  Solid  Free-hand  Geometrical  Drawing. .  1 2mo,  i  oo 

Drafting  Instruments  and  Operations i2mo,  i  25 

Manual  of  Elementary  Projection  Drawing i2mo,  i  50 

Manual  of  Elementary  Broblems  in  the  Linear  Perspective  of  Form  and 

Shadow i2mo,  i  oo 

Plane  Problems  in  Elementary  Geometry i2mo,  i  25 

Primary  Geometry i2mo,  75 

Elements  of  Descriptive  Geometry,  Shadows,  and^Perspective 8vo,  3  50 

General  Problems  of  Shades  and  Shadows 8vo,  3  oo 

Elements  of  Machine  Construction  and  Drawing 8vo,  7  SO 

Problems.  Theorems,  and  Examples  in  Descriptive  Geometry 8vo,  2  50 

Weisbach's  Kinematics  and  the  Power  of  Transmission.       v  Hermann  an-1 

Klein.)  8vo,  5  oo 

Whelpley's  Practical  Instruction  in  the  Art  of  Letter  Engraving i2mo,  2  oo 

Wilson's  Topographic  Surveying 8vo,  3  50 

Free-hand  Perspective : 8vo,  2  50 

Free-hand  Lettering.     (In  preparation.) 

Woolf's  Elementary  Course  in  Descriptive  Geometry Large  8vo,  3  oo 


'ELECTRICITY  AND   PHYSICS. 

Anthony  and  Brackett's  Text-book  of  Physics,  (Magic.) Small  8vo,  3  oo 

Anthony's  Lecture-notes  on  the  Theory  of  Electrical  Measurements 1 2mo ,  i  oo 

Benjamin'sIHistory  of  Electricity 8vo,  3  oo 

Voltaic  Cell 8vo,  3  oo 

Classen's  Quantitative  Chemical  Analysis  by  Electrolysis.  (Boltwood.).  .8vo,  300 

Crehore  and  Sauier's  Polarizing  Photo-chronograph 8vo ,  3  oo 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book. .  lomo,  morocco,  4  oo 

Flather's  Dynamometers,  and  the  Measurement  of  Power 12 mo,  3  oo 

Gilbert's  De  Magnete.  (Mottelay.) 8vo,  a  50 

Holman's  Precision  of  Measurements 8vo,  2  oo 

Telescopic  Mirror-scale  Method,  Adjustments,  and  Tests Large  8vo  75 

Lanaauer's  Spectrum  Analysis.  (Tingle.) 8vo,  3  oo 

Le  Chatelier's  High-temperature  Measurements.  (Boudouard — Burgess.  )i2mo,  3  oo 

Lob's  Electrolysis  and  Electrosynthesis  of  Organic  Compounds.  (Lorenz.)  i2mo,  i  oo 

*  Lyons's  Treatise  on  Electromagnetic  Phenomena.     Vols.  I.  and  11.  8vo,  each,^  61  oo 

*  Michie.     Elements  of  Wave  Motion  Relating  to^SoundJand  Light 8vo,  4  o_o 

Niaudet's  Elementary  Treatise  on  Electric  Batteries.     (FishoacK. ) 1 2010,  ™aT  lo 

*  Parshall  and  Hobart's  Electric  Generators. Small  4to.  half  morocco,  10  oo 

*  Rosenberg's  Electrical  Engineering.    (Haldane  Gee — Kinzbrunner.) 8vo,  i   50 

Ryan,  Norris,  and  Hoxie's  Electrical  Machinery.     (In  preparation-* 

Thurston's  Stationary  Steam-engines 8vo,  2  50 

*  Tillman's  Elementary  Lessons  in  Heat 8vo,  i  50 

Tory  and  Pitcher's  Manual  of  Laboratory  Physics Small  8vo,  2  oo 

Ulke's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 


LAW. 

*iDavis's  Elements  of  Law 8vo,  2  50 

*         Treatise  on  the  Military  Law  of  United  States 8vo,  7  oo 

Sheep,  7  SO 

Manual  for  Courts-martial i6mo,  morocco,  i  50 

9 


Wait's  Engineering  and  Architectural  Jurisprudence 8vo,  6  oo 

Sheep,  6  50 

Law  of  Operations  Preliminary  to  Construction  in  Engineering'and  Archi- 
tecture      8vo,  5  oo 

Sheep,  5  50 

Law  of  Contracts 8vo,  3  oo 

Winthrop's  Abridgment  of  Military  Law i2mo,  2  50 


MANUFACTURES. 

Bernadou's  Smokeless  Powder — Nitro-cellulose  and  Theory  of  the  Cellulose 

Molecule i2mo,  2  50 

Holland's  Iron  Founder i2mo,  2  50 

"  The  Iron  Founder,"  Supplement i2mo,  2  50 

Encyclopedia  of  Founding  and  Dictionary  of_Foundry  Terms  Used^in  the 

Practice  of  Mould.ing i2mo,  3  oo 

Eissler's  Modern  High  Explosives 8vo,  4  oo 

Effront's  Enzymes  and  their  Applications.     (Prescott.) 8vo,  3  oo 

Fitzgerald's  Boston  Machinist i8mo,  i  oo 

Ford's  Boiler  Making  for  Boiler  Makers i8mo,  i  oo 

Hopkins's  Oil-chemists'  Handbook. 8vo,  3  oo 

Keep's  Cast  Iron 8vo,  2  50 

Leach's  The  Inspection  and  Analysis  of  Food  with  SpecialjReference  to  State 

Control.     (In  preparation.) 

Metcalf's  Steel.     A  Manual  for  Steel-users i2mo,  2  oo 

Metcalfe's  Cost  of  Manufactures — And  the  Administration    of  Workshops, 

Public  and  Private 8vo,  5  oo 

Meyer's  Modern  Locomotive  Construction 4to,  10  oo 

*  Reisig's  Guide  to  Piece-dyeing 8vo,  25  oo 

Smith's  Press-working  of  Metals 8vo,  3  oo 

Wire:  Its  Use  and  Manufacture. '. .  .Small  4to,  3  oo 

Spalding's  Hydraulic  Cement i2mo,  2  oo 

Spencer's  Handbook  for  Chemists  of  Beet-sugar  Houses i6mo,  morocco,  3  oo 

Handbook  tor  sugar  Manufacturers  and  their  Chemists..  .  i6mo,  morocco,  2  oo 
Thurston's  Manual  of  Steam-boilers,  their  Designs,  Construction  and  Opera- 
tion  8vo,  5  oo 

*  Walke's  Lectures  on  Explosives 8vo,  4  oo 

West's  American  Foundry  Practice I2mo,  2  50 

Moulder's  Text-book i2mo,  2  50 

Wiechmann's  Sugar  Analysis Small  8vo,  2  50 

Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  oo 

Woodbury's  Fire  Protection  of  Mills 8vo,  2  50 


MATHEMATICS. 

Baker's  Elliptic  Functions 8vo,  i  50 

*  Bass's  Elements  of  Differential  Calculus i2mo,  4  oo 

Briggs's  Elements  of  Plane  Analytic  Geometry 12 mo,  oo 

Chapman's  Elementary  Course  in  Theory  of  Equations 1 2 mo,  50 

Compton's  Manual  of  Logarithmic  Computations i2mo,  50 

Da  vis's  Introduction  to  the  Logic  of  Algebra '. 8vo,  50 

*  Dickson's  College  Algebra Large  i2mo,  50 

*  Introduction  to  the  Theory  of  Algebraic  Equations    Large^i2mo,  25 

Halsted's  Elements  of  Geometry 8vo,  75 

Elementary  Synthetic  Geometry 8vo  50 

10 


*  Johnson's  Three-place  Logarithmic  Tables:    Vest-pocket  size paper,        15 

100  copies  for  5  oo 

*  Mounted  on  heavy  cardboard,  8  X  10  inches,         25 

10  copies  for  2  oo 

Elementary  Treatise  on  the  Integral  Calculus Small  8vo,  i  50 

Curve  Tracing  in  Cartesian  Co-ordinates i2mo,  i  oo 

Treatise  on  Ordinary  and  Partial  Differential  Equations Small  8vo,  3  5° 

Theory  of  Errors  and  the  Method  of  Least  Squares i2mo,  i  50 

*  Theoretical  Mechanics i2mo,  3  oo 

Laplace's  Philosophical  Essay  on  Probabilities.     (Truscott  and  Emory.)  i2mo,  2  oo 

*  Ludlow  and  Bass.     Elements  of  Trigonometry  and  Logarithmic  and  Other 

Tables , 8vo,  3  oo 

Trigonometry  and  Tables  published  separately Each,  2  oo 

Maurer's  Technical  Mechanics.     (In  preparation.) 

Merriman  and  Woodward's  Higher  Mathematics 8vo,  5  oo 

Merriman's  Method  of  Least  Squares 8vo,  2  oo 

Rice  and  Johnson's  Elementary  Treatise  on  the  Differential  Calculus . Sen.,  8vo,  3  oo 

Differential  and  Integral  Calculus.     2  vols.  in  one Gznall  8vo,  2  50 

Wood's  Elements  of  Co-ordinate  Geometry 8vo,  2  oo 

Trigonometry:  Analytical,  Plane,  and  Spherical i2mo,  i  oo 

MECHANICAL   ENGINEERING. 
MATERIALS  OF  ENGINEERING,  STEAM-ENGINES  AND  BOILERS. 

Baldwin's  Steam  Heating  for  Buildings I2mo,  2  50 

Barr's  Kinematics  of  Machinery 8vo,  2  50 

*  Bartlett's  Mechanical  Drawing 8vo,  3  oo 

Benjamin's  Wrinkles  and  Recipes i2mo,  2  oo 

Carpenter's  Experimental  Engineering 8vo,  6  oo 

Heating  and  Ventilating  Buildings 8vo,  4  oo 

Clerk's  Gas  and  Oil  Engine t* Small  8vo,  4  oo 

Coolidge's  Manual  of  Drawing 8vo,    paper,  i  oo 

Cromwell's  Treatise  on  Toothed  Gearing i2mo,  i  50 

Treatise  on  Belts  and  Pulleys i2mo,  i  50 

Durley's  Kinematics  of  Machines 8vo,  4  oo 

Flather's  Dynamometers  and  the  Measurement  of  Power 12 mo,  3  oo 

Rope  Driving I2mo,  2  oo 

Gill's  Gas  and  Fuel  Analysis  for  Engineers i2mo,  i  25 

Hall's  Car  Lubrication i  amo,  i  oo 

Button's  The  Gas  Engine.     (In  preparation.') 
Jones's  Machine  Design: 

Part   I. — Kinematics  of  Machinery 8vo,  i  50 

Part  II. — Form,  Strength,  and  Proportions  of  Parts 8vo,  3  oo 

Kent's  Mechanical  Engineer's  Pocket-book i6mo,    morocco,  5  oo 

Kerr's  Power  and  Power  Transmission 8vo,  2  oo 

MacCord's  Kinematics;  or,  Practical  Mechanism 8vo,  5  oo 

Mechanical  Drawing 4to,  4  oo 

Velocity  Diagrams 8vo,  i  50 

Mahan's  Industrial  Drawing.    (Thompson.) 8vo,  3  50 

Poole's  Calorific  Power  of  Fuels 8vo,  3  oo 

Reid's  Course  in  Mechanical  Drawing 8vo.  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design.  .8vo,  3  oo 

Richards's  Compressed  Air i2mo,  i  50 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Smith's  Press-working  of  Metals 8v«  3  oo 

Thurston's  Treatise  on    Friction   and    Lost  Work   in    Machinery   and   Mil 

Work 8vo,  3  oo 

Animal  as  a  Machine  and  Prime  Motor,  and  the  Laws  of  Energetics .  i2mo,  i  oo 

11 


Warren's  Elements  of  Machine  Construction  and  Drawing ?  vo,  7  50 

Weisbach's  Kinematics  and  the  Power  of  Transmission.      Herrmann — 

Klein.) 8vo,  5  oo 

Machinery  of  Transmission  and  Governors.     (Herrmann — Klein.).  ,8vo,  5  oo 

Hydraulics  and  Hydraulic  Motors.     (Du  Bois.) 8vo,  5  oo 

Wolff's  Windmill  as  a  Prime  Mover 8vo,  3  oo 

Wood's  Turbines 8vo,  2  50 

MATERIALS  OF  ENGINEERING. 

Bovey's  Strength  of  Materials  and  Theory  of  Structures 8vo,  7  50 

Burr's  Elasticity  and  Resistance  of  the  Materials  of  Engineering.     6th  Edition, 

Reset : 8vo.  7  50 

Church*s  Mechanics  of  Engineering Svo,  6  oo 

Johnson's  Materials  of  Construction Large  8vo,  6  oo 

Keep's  Cast  Iron 8vo  2  50 

Lanza's  Applied  Mechanics 8vo,  7  50 

Martens's  Handbook  on  Testing  Materials.     (Henning.) 8vo,  7  So 

Merriman's  Text-book  on  the  Mechanics  of  Materials 8vo,  4  oo 

Strength  of  Mater'als i2mo,  i  oo 

Metcalf's  SteeL     A  Manual  for  Steel-users i2mo  2  oo 

Smith's  Wire :  Its  Use  and  Manufacture Small  4to,  3  oo 

Materials  of  Machines v i2mo,  i  oo 

Thurston's  Materials  of  Engineering 3  vols. ,  Svo  8  oo 

Part   II.— Iron  and  Steel Svo,  3  So 

Part  in. — A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and  their 

Constituents Svo,  2  50 

Text-book  of  the  Materials  of  Construction Svo  5  oo 

Wood's  Treatise  on  the  Resistance  of  Materials  and  an  Appendix  on  the 

Preservation  of  Timber Svo,  2  oo 

Elements  of  Analytical  Mechanics Svo,  3  oo 


STEAM-ENGINES  AND  BOILERS. 

Carnot's  Reflections  on  the  Motive  Power  of  Heat.     (Thurston.) i2mo,  i  50 

Dawson's  "Engineering"  and  Electric  Traction  Pocket-book.  .i6mo,  mor.,  4  oo 

Ford's  Boiler  Making  for  Boiler  Makers i8mo,  i  oo 

Goss's  Locomotive  Sparks 8vo,  2  oo 

Hemenway's  Indicator  Practice  and  Steam-engine  Economy 12 mo,  a  oo 

Button's  Mechanical  Engineering  of  Power  Plants Svo,  5  oo 

Heat  and  Heat-engines Svo,  5  oo 

Kent's  Steam-boiler  Economy Svo,  4  oo 

Kneass's  Practice  and  Theory  of  the  Injector Svo.  i  50 

MacCord's  Slide-valves Svo,  2  oo 

Meyer's  Modern  Locomotive  Construction 4to,  10  oo 

Peabody's  Manual  of  the  Steam-engine  Indicator i2mo,  i  50 

Tables  of  the  Properties  of  Saturated  Steam  and  Other  Vapors Svo,  i  oo 

Thermodynamics  of  the  Steam-engine  and  Other  Heat-engines Svo,  5  oo 

Valve-gears  for  Steam-engines Svo,  2  50 

Peabody  and  Miller's  Steam-boilers 8vo,  4  oo 

Pray's  Twenty  Years  with  the  Indicator Large  Svo,  2  50 

Pupln's  Thermodynamics  of  Reversible  Cycles  in  Gases  and  Saturated  Vapors. 

(Osterberg.) i2mo,  i  25 

Reagan's  Locomotives :  Simple,  Compound,  and  Electric i2mo,  2  50 

Rontgen's  Principles  of  Thermodynamics.     (Du  Bois.) Svo,  5  oo 

Sinclair's  Locomotive  Engine  Running  and  Management i2mo,  2  oo 

Smart's  Handbook  of  Engineering  Laboratory  Practice i2mo,  2  50 

Snow's  Steam-boiler  Practice Svo,  3  oo 

12 


Spangler's  Valve-gears 8vo,    2  50 

Notes  on  Thermodynamics i2mo,    i  oo 

Spangler,  Greene,  and  Marshall's  Elements  of  Steam-engineering 8vo,    3  oo 

Thurston's  Handy  Tables 8vo.    i   50 

Manual  of  the  Steam-engine 2  vols..  8vo     10  oo 

Part  I.— History.  Structuce,  and  Theory 8vo,    600 

Part  n. — Design,  Construction,  and  Operation 8vo,    6  oo 

Handbook  of  Engine  and  Boiler  Trials,  and  the  Use  of  the  Indicator  and 

the  Prony  Brake 8vo     5  oo 

Stationary  Steam-engines 8vo,    2  5° 

Steam-boiler  Explosions  in  Theory  and  in  Practice I2mo,    i   50 

Manual  of  Steam-boilers  .Their  Designs,  Construction,  and  Operation.  8vo,    5  oo 

Weisbach's  Heat,  Steam,  a  -i j  Steam-engines.     (Du  Bois.) 8vo,    5  oo 

Whitham's  Steam-engine  I  esign .8vo,    5  oo 

Wilson's  Treatise  on  Steam-boilers.     (Flather.) i6mo,    2  50 

Wood's  Thermodynamics.  Heat  Motors,  and  Refrigerating  Machines.  .  .  .8vo,    4  oo 


MECHANICS    A.ND   MACHINERY. 

Barr's  Kinematics  ot  Machinery 8vo,    2~5O 

Bovey's  Strength  of  Materials  and  Theory  of  Structures 8vo,    7  50 

Chase's  The  Art  of  Pattern-making 12010,    2  50 

Chordal. — Extracts  from  Letters I2mo,    2  oo 

Church's  Mechanics  of  Engineering 8vo     6  oo 

Notes  and  Examples  in  Mechanics 8vo.    2  oo 

Compton's  First  Lessons  in  Metal-working i  amo,        50 

Compton  and  De  Groodt's  The  Speed  Lathe I2mo,        50 

Cromwell's  Treatise  on  Toothed  Gearing I2mo,        50 

Treatise  on  Belts  and  Pulleys i2mo,        50 

Dana's  Text-book  of  Elementary  Mechanics  -/or  the   Use  of  Colleges  and 

Schools i2mo,       50 

Dingey's  Machinery  Pattern  Making i zmo,        oo 

Dredge's   Record  of  the  Transportation   Exhibits  Building  of  the   World's 

Columbian  Exposition  of  1893 4to,  half  morocco,    5  oo 

Du  BoU's  Elementary  Principles  of  Mechanics : 

VoL     I. — Kinematics 8vo,    3  50 

Vol.   n.— Statics 8vo,   4  oo 

Vol.  HI. — Kinetics 8vo,    3  50 

Mechanics  of  Engineering.     VoL  I Small  4to,      7  50 

Vol.  H Small  4to,    10  oo 

Durley's  Kinematics  of  Machines 8vo,        oo 

Fitzgerald's  Boston  Machinist :6mo.        oo 

Flather's  Dynamometers,  and  the  Measurement  of  Power i  zmo,        oo 

Rope  Driving I2mo,        oo 

Goss's  Locomotive  Sparks 8vo,        oo 

Hall's  Car  Lubrication I2mo,        oo 

Holly's  Art  of  Saw  Filing i8mo         75 

*  Johnson's  Theoretical  Mechanics i2mo,    3  oo 

Statics  by  Graphic  and  Algebraic  Methods 8vo,    2  oo 

Jones's  Machine  Design: 

Part  I. — Kinematics  of  Machinery 8vo,    i  50 

Part  n. — Form,  Strength,  and  Proportions  of  Parts 8vo,    3  oo 

Kerr's  Power  and  Power  Transmission 8vo,    2  oo 

Lanza's  Applied  Mechanics 8vo.    7  50 

MacCord's  Kinematics;  or,  Practical  Mechanism 8vo,   5  oo 

Velocity  Diagrams 1 8ro,    i  50 

Maurer's  Technical  Mechanics,     (In  preparation.) 

13 


Merriman's  Text-book  on  the  Mechanics  of  Materials 8vo,  4  oo 

*  Michie's  Elements  of  Analytical  Mechanics 8sro,  4  oo 

Reagan's  Locomotives:  Simple,  Compound,  and  Electric i2mo,  2  50 

Reid's  Course  in  Mechanical  Drawing 8vo,  2  oo 

Text-book  of  Mechanical  Drawing  and  Elementary  Machine  Design.  .8vo,  3  oo 

Richards's  Compressed  Air i2mo,  i  50 

Robinson's  Principles  of  Mechanism 8vo,  3  oo 

Ryan,  Norris,  and  Hoxie's  Electrical  Machinery.     (In  -preparation.) 

Sinclair's  Locomotive-engine  Running  andfManagement i2mo,  2  oo 

Smith's  Press-working  of  Metals 8vo,  3  oo 

Materials  of  Machines i2mo,  i  oo 

Spangler,  Greene,  and  Marshall's  Elements  of  Steam-engineering 8vo,  3  oo 

Thurston's  Treatise  on  Friction  and  Lost  Work  in  Machinery  and   Mill 

Work 8vo,  3  oo 

Animal  as  a  Machine  and  Prime  Motor,  and  the  Laws  of  Energetics .  i2mo,  i  oo 

Warren's  Elements  of  Machine  Construction  and  Drawing 8vo,  7  50 

Weisbach's    Kinematics  'I  and    the   Power  of    Transmission.     (Herrmann — 

Klein.) 8vo,  5  oo 

Machinery  of  Transmission  and  Governors.     (Herrmann — Klein. ).8vo,  5  oo 

Wood's  Elements  of  Analytical  Mechanics 8vo,  3  oo 

Principles  of  Elementary  Mechanics i2mo,  i  25 

Turbines 8vo,  2  50 

The  World's  Columbian  Exposition  of  1893 4to,  i  oo 

METALLURGY. 

Egleston's  Metallurgy  of  Silver,  Gold,  and  Mercury: 

Vol.   I.— Silver 8vo,  7  50 

Vol.   II.— Gold  and  Mercury 8vo,  7  So 

**  Iles's  Lead-smelting.     (Postage  9  cents  additional.) i2mo,  2  50 

Keep's  Cast  Iron 8vo,  2  50 

Kunhardt's  Practice  of  Ore  Dressing  in  Europe 8vo,  i  50 

Le  Chatelier's  High-temperature  Measurements.   (Boudouard — Burgess.) .  i2mo,  3  oo 

Metcalf 's  Steel.     A  Manual  for  Steel-users i2mo,  2  oo 

Smith's  Materials  of  Machines i2tno,  i  oo 

Thurston's  Materials  of  Engineering.     In  Three  Parts 8vo,  8  oo 

Part  II.— Iron  and  Steel 8vo,  3  50 

Part  III. — A  Treatise  on  Brasses,  Bronzes,  and  Other  Alloys  and   their 

Constituents 8vo,  2  50 

Ulke's  Modern  Electrolytic  Copper  Refining 8vo,  3  oo 

MINERALOGY. 

Barringer's  Description  of  Minerals  of  Commercial  Value.     Oblong,  morocco,  2  50 

Boyd's  Resources  of  Southwest  Virginia 8vo,  3  oo 

Map  of  Southwest  Virginia Pocket-book  form,  2  oo 

Brush's  Manual  of  Determinative  Mineralogy.     (Penfield.) 8vo,  4  oo 

Chester's  Catalogue  of  Minerals 8vo,  paper,  i  oo 

Cloth,  i  25 

Dictionary  of  the  Names  of  Minerals 8vo,  3  50 

Dana's  System  of  Mineralogy Large  8vo,  half  leather,    12  50 

First  Appendix  to  Dana's  New  "System  of  Mineralogy.".  . .  .Large  8vo,  i  oo 

Text-book  of  Mineralogy 8vo,  4  oo 

Minerals  and  How  to  Study  Them i2mo,  i  50 

Catalogue  of  American  Localities  of  Minerals Large  8vo,  i  oo 

Manual  of  Mineralogy  and  Petrography i2mo,  2  oo 

Egleston's  Catalogue  of  Minerals  and  Synonyms 8vo ,  2  50 

Hussak's  The  Determination  of  Rock-forming  Minerals.     (Smith.)  Small  8vo,  2  oo 

14 


*  Penfield's  Notes  on  Determinative  Mineralogy  and  Record  of  Mineral  Tests. 

8vo,  paper,    o  50 
Rosenbusch's   Microscopical   Physiography   of   the    Rock-making   Minerals. 

(Iddings.) 8vo,  5  oo 

*  Tillman's  Text-book  of  Important  Minerals  and  Docks 8vo,  2  oo 

Williams's  Manual  of  Lithology 8vo,  3  oo 

HUNTING. 

Beard's  Ventilation  of  Mines I2mo,  2  50 

Boyd's  Resources  of  Southwest  Virginia 8vo,  3  oo 

Map  of  Southwest  Virginia Pocket-book  form,  2  oo 

*  Drinker's  Tunneling,  Explosive  Compounds,  and  Rock  Drills. 

4to,  half  morocco,  25  oo 

Eissler's  Modern  High  Explosives 8vo,  4  oo 

Fowler's  Sewage  Works  Analyses I2mo,  2  oo 

Goodyear's  Coal-mines  of  the  Western  Coast  of  the  United  States I2mo,  2  50 

Ihlseng's  Manual  of  Mining 8vo,  4  oo 

**  Iles's  Lead-smelting.     (Postage  oc.  additional.) i2mo,  2  50 

Kunhardt's  Practice  of  Ore  Dressing  in  Europe 8vo,  i  50 

O'Driscoll's  Notes  on  the  Treatment  of  Gold  Ores 8vo,  2  oo 

*  Walke's  Lectures  on  Explosives 8vo,  4  oo 

Wilson's  Cyanide  Processes I2mo,  i  50 

Chlorination  Process i2mo,  i  50 

Hydraulic  and  Placer  Mining I2mo,  2  oo 

Treatise  on  Practical  and  Theoretical  Mine  Ventilation i2mo,  i  25 


SANITARY  SCIENCE. 

Copeland's  Manual  of  Bacteriology.     (In  preparation.) 

FolwelTs  Sewerage.     (Designing,  Construction,  and  Maintenance. ) 8vo,  3  oo 

Water-supply  Engineering 8vo,  4  oo 

Fuertes's  Water  and  Public  Health 1 2mo,  i  50 

Water-filtration   Works 1 2mo ,  2  50 

Gerhard's  Guide  to  Sanitary  House-inspection i6mo,  i  oo 

Goodrich's  Economical  Disposal  of  Town's  Refuse .Demy  8vo,  3  50 

Hazen's  Filtration  of  Public  Water-supplies 8vo,  3  oo 

Kiersted's  Sewage  Disposal I2mo,  i  25 

Leach's  The  Inspection  and  Analysis  of  Food  with  Special  Reference  to  State 

Control.     (In  preparation.) 

Mason's    Water-supply.     (Considered   Principally   from    a    Sanitary    Stand- 
point.)    3d  Edition,  Rewritten 8vo,  4  oo 

Examination  of  Water.     (Chemical  and  Bacteriological.) 12 mo,  i  25 

Merriman's  Elements  of  Sanitary  Engineering. 8vo,  2  oo 

Nichols's  Water-supply.     (Considered  Mainly  from  a  Chemical  and  Sanitary 

Standpoint. )     (1883.) 8vo,  2  50 

Ogden's  Sewer  Design 12 mo,  2  oo 

*  Price's  Handbook  on  Sanitation i2mo,  i  50 

Richards's  Cost  of  Food.     A  Study  in  Dietaries i2mo,  i  oo 

Cost  of  Living  as  Modified  by  Sanitary  ^Science 12  mo,  i  oo 

Richards  and  Woodman's  Air,  Water,  and  Food  from  a  Sanitary  Stand- 
point   8vo,  2  oo 

*  Richards  and  Williams's  The  Dietary 'Computer 8vo,  i  50 

Rideal's  Sewage  and  Bacterial  Purification  of  Sewage 8vo,  3  50 

Turneaure  and  Russell's  Public  Water-supplies 8vo,  5  oo 

Whipple's  Microscopy  of  Drinking-water 8vo,  3  50 

Woodhull's  Notes  and  Military  Hygiene i6mo,  i  50 

15 


MISCELLANEOUS. 

Barker's  Deep-sea  Soundings 8vo,  2  oo 

Emmons's  Geological  Guide-book  of  the  Rocky  Mountain  Excursion  of  the 

International  Congress  of  Geologists Large  8vo,  i  50 

Ferrel's  Popular  Treatise  on  the  Winds 8vo,  4  oo 

Haines's  American  Railway  Management i2mo,  2  50 

Mott's  Composition.'Digestibility,  and  Nutritive  Value  of  Food.    Mounted  chart,  i  25 

Fallacy  of  the  Present  Theory  of  Sound i6mo,  i  oo 

Ricketts's  History  of  Rensselaer  Polytechnic  Institute,  1824-1894.  Small  8vo,  3  oo 

Rotherham's  Empnasized  New  Testament Large  8vo,  2  oo 

Steel's  Treatise  on  the  Diseases  of  the  Dog 8vo,  3  50 

Totten's  Important  Question  in  Metrology 8vo,  2  50 

The  World's  Columbian  Exposition  of  1893 4to,  i  oo 

Worcester  and  Atkinson.     Small  Hospitals,  Establishment  and  Maintenance, 
and  Suggestions  for  Hospital  Architecture,  with  Plans  for  a  Small 

Hospital I2mo,  i  25 

HEBREW  AND  CHALDEE  TEXT-BOOKS. 

Green's  Grammar  of  the  Hebrew  Language 8vo,  3  oo 

Elementary  Hebrew  Grammar i2mo,  i  25 

Hebrew  Chrestomathy 8vo,  2  oo 

Gesenius's  Hebrew  and  Chaldee  Lexicon  to  the  Old  Testament  Scriptures. 

(Tregelles.) Small  4to,  half  morocco,  5  oo 

Letteris's  Hebrew  Bible 8vo,  2  25 

16 


THIS  BOOK  IS  DUE  ON  THE  LAST  DATE 
J'  STAMPED  BELOW 

AN  INITIAL  FINE  OF  25  CENTS 

WILL  BE  ASSESSED  FOR  FAILURE  TO  RETURN 
THIS  BOOK  ON  THE  DATE  DUE.  THE  PENALTY 
WILL  INCREASE  TO  SO  CENTS  ON  ™E  FOURTH 
DAY  AND  TO  $I.OO  ON  THE  SEVENTH  DAY 
OVERDUE. 


JAN    10 


LD  21-100m-7,'39(402s) 


